Neil Stockwell is an unlikely looking film star. He is one of those men who give the impression of being at least as wide as he is tall; he wears a roomy woollen hat as he bags up apples and pears in front of the stall that his family has held for four generations in Queen's Market in Newham, one of the poorest corners of East London. Even so, next month, Stockwell should be coming to a cinema near you to steal the show in American documentary maker Robert Greenwald's terrific film Wal-Mart: The High Cost of Low Price.

In truth, Stockwell tells me, he would much rather not have made his Hollywood debut, but he is fighting for his life. He has worked at Queen's Market with his brother for the last 35 years, having taken over from their grandad, who put in 70 years, before he died at 88. Stockwell gets up four days a week before most of us have gone to bed to buy his fruit at Spitalfields, a couple of miles up the road. He then works all day to shift as much of it as he can to possibly the most multi-cultural clientele anywhere in the world. In an average week, he reckons to sell about 18 tons of fruit, easy.

These days he can move anything: papaya, mangoes, lychees, longan fruit, guava. The more diverse the market gets, the better his business becomes. 'There's such a big influx of Asian people,' he says, 'and they love their fruit. When we were kids you went to see your grandad on a Sunday and you took him 20 Player's and a small bottle of Scotch. With the Asian community, they'll say: "Neil I'm going to see my grandad, can I have a box of apples, a box of oranges and a box of bananas." No wonder they live so long.'

It's a Tuesday when I meet Neil and he is clearing the decks for the busy end of the week. Today you can get 25 satsumas for a pound, or 15 apples. Women, some in jeans, some in saris, some in shalwar kameez, line up to buy. No one can beat him on price or freshness or range, least of all the supermarket that is looming over his future, but none of that will matter if the council gets its way. The site of Queen's Market is being sold to a city developer, St Modwen, owner of Longbridge and Elephant & Castle among other properties. In a £75 million deal St Modwen plans to put the entrance to an Asda mall where Neil has his pitch: hence the battle for his life and his burgeoning film career.

'The truth of the matter is,' he says, 'Asda have come here and they have seen how busy the market is, they've got their scouts, and they have gone to the council, through these middlemen, and said: right we'll develop that site for you. They talk about regeneration and so on, they're building 200 flats that no one here can afford, and the council sees the pound note signs. I've been told by the New Labour mayor, Sir Robin Wales, to my face, that what we do here is "second class" and that he is looking for a "better class" of environment. We have given them 12,000 names on a petition of people who want to keep the market. They have not given us one in return that says they want a Wal-Mart Asda.'

Neil Stockwell's battle is a new front line in a larger campaign that is documented in Robert Greenwald's film, which builds a picture of the devastating impact Wal-Mart culture has had on the landscape of small-town America, and is having on the wider world. Greenwald, whose previous film, about Murdoch's Fox News, Outfoxed, earned him a reputation for polemic, shows how the homey store created by Sam Walton in the Fifties, now the world's wealthiest corporation with a revenue of nearly $300 billion, blights almost every aspect of American life.

It is hard to say which part of the film is most shocking. There are the interviews with the owners of father and son stores in middle America who watch their life's work destroyed overnight by the arrival of a Supercenter. There is the footage from squalid Chinese sweatshops, where workers live in slave conditions to fuel the aggressive discounting that Wal-Mart (slogan 'Every Day Low Prices') prides itself in; there are the testimonies of former store managers forced to cut overheads every month by understaffing and underpaying and denying health cover to 'associates' (which is what they call the workforce); there are the antiunion Swat teams that fly in on private jets from headquarters in Arkansas to target and harass any individuals who make a motion to join a union; and there is the unbelievable litany of murders and rapes and muggings that have occurred in Wal-Mart car parks because the company refuses to shell out on security.

Even among all of this, it is the vast hypocrisies that hit home. Greenwald structures his film around a rabble-rousing speech given by Wal-Mart CEO Lee Scott about how the company 'does the right thing' for its workers and its suppliers, and yet still manages to grow: 'We had record sales this year! We had record earnings! You better be ready to be better!' Scott boasts how, when on the road, he shares a room with his chief finance officer to save the company a hundred dollars.

He does not, however, mention the fact that he earns $27 million a year, while a large percentage of full-time Wal-Mart 'associates' exist well below the poverty line. He does not mention the fact that the Walton family, Sam Walton's widow and their children, occupy five places in the top 10 wealthiest people in America and that their collective fortune is double that of Bill Gates. And he does not mention the fact that when the company's employees set up a fund to help fellow workers who had fallen on hard times, through natural disaster, and contributed $5 million to it, the Walton family came up with a combined $6,000.

I spoke to Robert Greenwald about the impact his film has had since its release in America at the end of last year, where it did not make the multiplexes but was shown in a nationwide network of schools and churches and town halls that he has established with previous projects. Wal-Mart itself, he suggests, has reacted as you imagine it would: 'They have attacked me very viciously, personally and quite stupidly,' he says. 'They put out a 10- or 15-page brochure which contained every bad review I have ever had; they hired operatives to rubbish me.' But, hopefully, he believes that they have put the company on the defensive at least. 'The film is part of a movement. I think we have begun something, but whether the corporate culture will allow Wal-Mart and the other big retailers to be anything other than pathological in their response is unclear. The problem in America is that corporations have funded an awful lot of politicians one way or another, and they are not all Republicans.'

He says he got intrigued with the idea of having Lee Scott in the movie, even though Wal-Mart refused to co-operate with it. 'In a way it is better to see him in his element,' he says. He sees the film as another way of analysing the question asked by Thomas Frank in his best selling book What's the Matter with America? Greenwald, like Frank, was trying to fathom in part why the poor and unemployed of middle America continued to elect a party and a president so intent on promoting corporate interests at the expense of their own. Wal-Mart, a company that has built its profits on 'giving China a better distribution centre' at the heart of the States, sells itself as being American as Mount Rushmore, and America buys it. 'They are,' like most corporations, Waldman suggests, selling a lifestyle: 'you will feel better, more American, if you buy from Wal-Mart'.

I wonder if he thinks that Wal-Mart - which if it were a country, would have the 20th biggest GDP on the planet - is content to dominate its home market or whether it eyes the world?

'It's definitely world domination they want,' Greenwald says. 'They have done America: we have 4,000 Wal-Marts already. They're looking to Europe and to China in particular.'

Greenwald sees films like his own as a small gesture against the tide, but an important one nevertheless. 'I believe that social movements take the lead and the politicians follow. I think we will see the same with the anti-corporate movement, at least I hope so. I think the more people who look at the way Wal-Mart is run and owned, they will see it is greed run amok.'

Given the vested interests he is up against I wonder if Greenwald has ever got paranoid about his personal safety. 'It hasn't gone that far,' he says, laughing a bit. 'But put it this way: I always look both ways these days before I cross the road.'

The day after I spoke to Greenwald, oddly, I came across Lee Scott's name again. Gordon Brown had appointed the Wal-Mart leader as one of a group of 'wise men' to advise on how Britain should shape its economy for globalisation. Getting Scott to advise on globalisation seems a little like asking Donald Rumsfeld to offer quiet thoughts on diplomacy, but then, given the hold supermarket culture seems to have over the government, nothing is a surprise.

The mechanics of this unholy alliance finds a nice little microcosm in Newham. The ease with which Wal-Mart Asda is being allowed to occupy the High Street in West Ham is mirrored in high streets throughout the country, where Tesco and Sainsbury's and the rest are routinely welcomed to prime sites at the expense of local competition. Asda currently has £400 million-worth of sites waiting for planning consents. Queen's Market is one of at least 60 traditional street markets that are currently under threat from the arrival of the developers. Somehow, planning departments seem to have got the idea of 'think globally, act locally' the wrong way round.

The rearguard action for the traders in Newham in the face of the world's biggest supermarket is being organised by a group called Friends of Queen's Market. I met Claire Peasnall, one of its driving forces, in a halal café beside the stalls. She moved here from the west of London principally for the market. She has her maps and plans and petitions spread out on the table, but she speaks from the heart about the idea of markets, the organic process by which they have grown to cater to every need and whim of the local population, the way they provide a little social-welfare system of their own: 'The stall holders all give stuff away at the end of the day. They know who all the poorest people are. And then other guys come in and buy up what's left and sell it for even less. So in a curious way everyone gets fed.'

Much of her scorn is reserved for the New Labour mayor Sir Robin Wales (christened Sir Robin Wal-Mart by Private Eye), who has the right of veto over his 60 elected councillors, 59 of whom, all Labour, vote with him anyway. Wales imagines a shopping experience on the site to 'rival Bluewater'. The mayor says he is not taking the market away, he is improving it. This improvement will move it to an inferior, smaller site adjacent to the Asda store and on a different level, without a carpark, and leave it to the mercy of the developer. The stallholders don't hold out much hope. Nick Kaye of St Modwen has publicly told traders that markets like Queen's are finished. 'Your market is crap,' he reportedly suggested, 'and suffers from too many veg stalls and an excess of meat shops.' The small shops that line the market have been informed by St Modwen that they face rent rises of 300 per cent. None of them will survive that; nearly all are run by Asian families.'It's racist in effect,' Peasnall says.

She shows me an artist's impression of the Asda mall. The shoppers have jackets slung over their shoulders taking in the East London sun. It could be Milan; and there is not a black or brown face among them. When Peasnall pointed out the fact that St Modwen were envisaging a wholly white community in a 90 per cent ethnic minority area, the artists impressions were redrawn with some of the faces coloured in.

As we talk, Danny Woodards, whose family have had stalls here for nearly a century, and who is an original East Ender with a big flat cap and a yard, and horses, wanders in. He is Neil Stockwell's fruit and veg rival. He talks of how the market is already practising a version of globalisation, and one far more user-friendly than that promoted by Wal-Mart Asda. 'What people don't realise is that the women who come here shop to cook. Ninety per cent of them don't work, but they do know how to cook. They will buy 10 kilos of onions, 10 kilos of potatoes, just for the week. They don't want any convenience food. We are told repeatedly that is what the government wants: home cooks. They're all driving me mad this morning, mind. I got a pound a bag for 10 kilos of onions. I got cauliflowers big as your head, three for a pound. A lousy knicker. And they stand there and wonder if they can have four or something.'

George Galloway was down here last Saturday, and lifelong Labour men like Danny Woodards are thinking about voting Respect, who have pledged to save the market. 'I had a bowl of milk waiting for George round the back, mind.' At a public meeting the previous night Woodards had said to Sir Robin Wales: do what you have to do, but retain the council running the market. 'He said they hadn't got the expertise. I said what you think you've been fucking doing the last 120 years?'

Claire Peasnall is decidedly not a fan of the mayor's. 'He's the sort of chap who you can imagine at school taking you round the back of the playing fields and twisting your ear,' she suggests to Danny.

'He wouldn't have twisted my ear because I'd have tore his head off, Claire, but I take your point.'

Before writing this piece I'd never set foot in an Asda, not on any kind of ethical grounds but simply because I have an inbuilt dislike of the idea of all that volume of stuff , all in one place, and I've never quite bought the idea that the possibility of a pair of jeans for £3 in a huge supermarket doesn't involve some kind of misery along the way. After I'd been at Queen's Market a while, though, I took a bus up to Becton, two miles away where there is already a big Asda superstore and a car park for a thousand cars. On the way I read the section of Joanne Blythman's brilliantly researched book Shopped: The Shocking Power of British Supermarkets, in which she signs up for an Asda induction programme. The bit that stays with me along with the 'smiley squad stickers' and the 'Pockets of Pride' incentives is the Miles of Smiles wall where new recruits are invited to pledge to The small shops that line the market face rent rises of 300 per cent. None of them will survive that customers better service in the future in a Maoist spirit of self-criticism: 'I will treat customers as I wish to be treated myself,' offers Lucy in checkout. 'I aim to give customers the best service ever,' says Mohammed in produce.

Blythman, in some ways, had it easy. When the American writer Barbara Ehrenreich did this exercise at an American Wal-Mart, she failed on a questionnaire designed to root out union activists: she was told that she had given a wrong answer when she merely agreed 'strongly' with the proposition that 'rules have to be followed to the letter at all times'. The only acceptable answer for Wal-Mart was 'very strongly'. Likewise, the only correct answer to the proposition 'there is room in every corporation for a non-conformist' was: 'totally disagree.'

At Becton the Asda 'colleagues' (as they are known in Britain) seem happy enough to help, but it is hard not to notice the difference between someone who has been incentivised into 'welcoming you' and someone who does so because they want to. I crave a little honest sullenness. I wander the aisles for a while, beset by a familiar kind of superstore despond, take notes of prices which compare unfavourably with those at the market, and hope in vain to see a couple of colleagues chatting to each other, which is considered 'time theft' by Wal-Mart HQ.

There is something in all vast supermarkets' relentless demonstration of choice that seems to suggest the opposite. Asda colleagues may have pockets of pride and so on but, in among the deals of the week, I find it's hard to get a few headlines out of my head. The one about the store being fi ned nearly a million pounds in the North East because it attempted to bribe employees to leave their union. The one about the store in Leicestershire where, in a panic about illegal immigrants, any colleague with an 'odd-sounding name', even those who had worked nearly 20 years at Asda, was told over the store tannoy to report to the manager's office with their papers. I collected a basket of shopping, but didn't fancy the queue, so I headed back to Queen's Market.

At the end of next month Ken Livingstone will launch his Food Strategy for London, which proposes itself as a blueprint for all cities in creating a healthier grocery and eating culture. Livingstone, with typical self-aggrandisement, is already calling it the most radical overhaul of Londoners' diet and health since the establishment of the welfare state. He has pledged to cut food miles with the help of 'prohibitively high' charges for polluting juggernauts and to end 'food deserts' in the poorest areas, where there are whole estates 'where you cannot buy a single piece of fresh food'. In his blueprint for this initiative the London mayor acknowledges the character and diversity that markets lend to the capital and stresses their role in providing access to healthy, fresh food that even the capital's most disadvantaged citizens can afford.

Like many such initiatives, this all sounds great until you see what is happening at places like Queen's Market, where the council forces through a scheme that to everyone working at the market, and nearly all those who use it, seems designed to destroy it. Jenny Linford, author of Food Lovers' London, can't get her head around this. 'Because of the multi-ethnic nature of the community, people are buying there every day - cheap food, fresh food, that they can cook from scratch. It's amazingly diverse - English traders next to Caribbean traders, African traders next to Asian traders. There are ingredients there that I've never even seen before! What kind of supermarket is going to stock five different kinds of plantain - yellow, black, green - with all their different uses?'

The council may be happy to describe Asda as 'an established Yorkshire-based company that has developed several community initiatives and is one of the main sponsors of the Children in Need appeal', but when it claims the new arrangement will give shoppers greater choice it is on very dodgy ground.

For Manish Patel, who runs an extraordinary Aladdin's cave of a hardware shop at the market, and who organises the traders' association opposed to the bill, the die is already cast. He imagined himself working here another 30 years 'until his two little girls are married' but already he is realising that life is going to be much more diffi cult. The warning signs have been given. In the past week he has had four parking tickets on a van that he has parked in the same place to unload for the past 20 years. He knows that the council and the developer and Asda want him out of his shop, just as surely as he knows that his customers want him here. He is not quite so sure where to look for hope. 'Maybe I can find a place in London where there is no Tesco or Asda and set up there, but I doubt it,' he says.

Neil Stockwell sees the same indications that it is the beginning of the end for him. 'We've been told we have to close now at six o'clock for example,' he says. 'We used to go on, on a summer evening, as long as we wanted. Me and my grandad used to drive out of here at midnight. Lovely summer's evening we'd be selling grapes, oranges, lemons. For some reason Asda will be allowed to stay open all night. Explain that to me?'

Robert Waldman ends his film on a note of optimism, with a little tribute to the growing number of small towns in America that have fought the arrival of Wal-Mart and won their battle. The traders at Queen's Market are happy to fight, and they put some of their faith in Ken Livingstone coming to their aid, but also they seem to feel that the battle is too big. 'I'm a barrow boy born and bred,' Neil Stockwell says. 'The day they take this stall away from me I don't know what will happen. But how can you fight the mayor?' he wonders. 'How can you fight an Asda?'

· Wal-Mart: The High Cost of Low Price is out in May

How the supermarkets are consuming Britain

1,306 the number of convenience stores currently owned by the Big Four (Sainsbury's, Asda, Tesco, Morrisons).
54 the number of convenience stores owned by the Big Four in 2000.

22% the percentage by which independent stores in the UK declined between 2001 and 2005.
40% the percentage of Britain's small independent grocery stores predicted to be lost by 2015.

319 the number of supermarket sites owned by the Big Four which remain undeveloped.
58% the percentage of those sites owned by Tesco. This number allegedly accounts for 4.5 million sq ft, which if developed could give it control of some 45% of the market.

75 the number of undeveloped sites which have been sold by supermarkets and are subject to restrictive covenants stipulating that they must not be used for grocery retailing once sold, usually for a minimum of two years, at least one for 125 years.

8 years the average age of the undeveloped sites, which has prompted the Office of Fair Trading to question whether such land is being held simply to prevent it falling into a rival's hands.

$8,463 the amount of money earned per hour by Wal-Mart CEO Lee Scott.
$9.68 the amount of money earned per hour by the average Wal-Mart worker.

1,713 the number of Wal-Marts in the US today. 3,131 the number they would like to have in the US by 2010.

3.1 million people work in the UK retail sector.
10% of whom are employed by Tesco.

£2.2 billion Tesco's latest profits.
1% the amount Tesco give of pre-tax profits to good causes.

£297 million Morrisons' 2005 profits. 0.044 % the percentage of their profits which they gave to charity in 2004/05.

£850,000 the amount Asda were fined by a British court this February for offering illegal inducement to try and get workers to leave their unions.
1.6 million the number of women who have sued Wal-Mart in a US court for gender discrimination, the largest class action suit in history.

5 the number of members of the Walton family, founders of Wal-Mart, who feature in the list of the top ten wealthiest people in America.

2 years the time expected it will take the Competition Commission to investigate the British grocery sector after the OFT referred it for examination, as it fears aspects of the sector (particularly supermarkets) 'distort competition and harm consumers'.