I'm chopping an onion for the tomato and beef ragu and shouting happily to Ruth: 'This and a bit of olive oil are the only things in this entire meal that were bought!' But she sniffs and says: 'Have you really thought through the ethics of this?'

For a journalist that question is never good news. Especially from your wife. So I reply, just a little bit spikily: 'You mean that this food is stolen?' I turn to the asparagus - beautiful, grown in Hereford, in perfect condition - and start cutting the stubs off it, though it's so fresh I hardly need to. 'Actually I don't consider it stolen - it's liberated.' I look at the strawberries, stacks of them, raised in Perthshire without a blemish. 'Liberated from those who neither wanted nor deserved it. Diverted on the way to the landfill.' Now I'm quoting my new friend Dave, an anarchist-freegan with whom I spent part of last night in a wheelie-bin outside Marks & Spencer.

Ruth looks at me with scorn. 'No, I don't mean that it's stolen. I mean that you're asking five adults and four small children to lunch and you're going to feed them food out of a rubbish bin. And you haven't even told them.'

I hadn't thought of it that way. Telling them would clearly spoil the surprise. The main reason I like cooking is for surprises ('You thought that was shellfish? It's snails, seaweed and garlic!') and this was going to be a classic. These three courses - grilled organic asparagus and courgettes with roasted cherry tomatoes; mince and tatties (new, organic) with a rocket-driven salad topped with caramelised peppers; strawberry fool, home-made raspberry yoghurt ice cream - had cost a total of 45p. I'd pictured a jolly scene at the end of the meal when I asked my friends to guess how I'd done it for just 5p a head and then watched their faces as I revealed that, apart from some herbs from my balcony and a lone onion, every single thing they'd just eaten had come from the bins behind M&S.

'They'll be gobsmacked,' I told my wife.

'They'll be nauseated.'

'They'll think it's brilliant.'

'Until the children start to vomit.'

It was different inside the wheelie-bin last night. With Dave and his friends - Pen, Cuthbert and Mike - there was a sense of ecstatic, naughty fun comparable to the time I broke into the school tuck shop. They have been 'bin-diving' every couple of nights for five months, but each time, they say, it's still like Christmas Day. And, as with all good hunter-gathering, it makes you hungry. There's no rule against grazing while you bin-dive, so we paused for a minute to sample a pack of Sushi to Share (£6) by the light of our bicycle torches. It wasn't very good - it was supermarket sushi. But it wouldn't be out of date for another half-an-hour. Much better was the 'hand-decorated chocolate birthday cake' (£7.99) we ate next.

'It's a bit of an advert for Marks and Sparks, isn't it?' said Cuthbert, a computer programmer specialising in freeware, and thus not exactly rich. 'I mean, if the M&S bins are OK, you really don't need to visit any other supermarket.' Their bins were, everyone agreed, prime ones. 'Miles better,' said Pen.

We'd arrived in a July drizzle on bicycles: the freegans won't travel any other way, not wishing to add carbon miles to those already racked up by the food. We scooted through the security barriers behind a Marks & Spencer in the suburbs of a northern city (I'm not allowed to reveal which one: the freegans need to protect their sources). Beside a loading bay, under the eye of a security camera, we found three big bins. The first was empty. The second, full of packaging and plastic. Opening the third we saw we'd hit the jackpot - a wheeled skip full to its five-foot high rim.

It was clear that we had got to the bin first. Sometimes, not often, the friends meet other bin-divers: there's usually a polite 'No, no, you first!' exchange. Freegans are decent people, and, in this shopping centre, at least, there's never been less than enough to go round. Often the security guards come down to say a friendly hello to the bin-divers: they, in turn, are scrupulous about not leaving any mess.

We started to sort through the rubbish, discarding what no one wanted into an empty bin. There was no question of throwing out the inedible food - there wasn't any. Nothing damaged or rotten at all, just packets that would be beyond their 'display until' date at midnight. So the selection process was sophisticated: 'Don't bother with those tomatoes - there's organic over here,' advised my colleagues, and 'Take this beef. It's Aberdeen Angus!' And the food was gorgeous - this was Marks & Spencer, after all, and it generally seemed that the most expensive stuff had been thrown out. 'It's a very good bin, this,' said someone with the air of a connoisseur.

So, this being the wettest July in 50 years, with shoppers uninterested in salads, we found an awful lot of beautiful lettuce, wild rocket and watercress as well as organic celery, peppers and thousands of tomatoes. There were 'biggest, sweetest cherries' from Turkey at an amazing £7.99 for a 350g box, and punnets of prime, British-grown strawberries. The only thing not in packets was perfect little bunches of Hereford asparagus - grown by the Chinn Family, the label said. I grabbed them. There were whole roast chickens and bags of cinnamon and raisin bagels, pizzas and yoghurts, a dozen packets of Filo Pastry Parcels Stuffed with Wild Mushroom. There were lots of M&S's posh ready meals - from the Gastropub range and others: I pondered a box of Cook! Fish Orkney Crab and Wild Rocket Bruschetta (£4.99) for a moment, but then gave it up to Dave. 'We've been doing this for five months,' he said. 'I'm eating better than I ever have in my life.'

When we were three-quarters of the way down the bin it was too difficult to reach in. So I volunteered to climb. Inside it I found myself up to my ankles in a sea of chicken breast fillets, sushi, chicken in tomato and basil, 'Children's Mild Curry', baked potatoes, potato pancakes, organic tomatoes from the Isle of Wight, mushroom in crème fraîche tartlets, coronation chicken salad. The bin smelt fine but I suddenly felt ill, sick at the waste: I couldn't help but tread on the stuff. All that immaculate fruit from hard-pressed British farmers squelching under my boots. I was glad when Cuthbert announced: 'I've got what I need - all else is greed.' And so we cycled off, each of us with a big rucksack full. Over the months Cuthbert's racing bike has developed bends in the back struts from the load.

At Dave's flat I saw the freegans for the first time in full light: for students they were pretty healthy - clear-skinned and a couple of them almost plump. We unpacked the rucksacks, and laid the haul out on the living-room floor. It was impressive - a back-of-an-estate-car shop - and the four friends expected to feed nine people, themselves and their flatmates, for two days out of the pile. So, crudely calculated, if we'd taken 10 per cent of the bin's contents, then the whole of it could have fed 180 people for a day. The thought made us all pause, the only sound Pen's contented munching on an Oriental style Deli Salad of Edamame and Soya beans with Chilli and Coriander dressing (£1.99).

'You could feed a village with this,' said Dave, who's spent time in southern Africa working as a volunteer in primary schools. It is wicked, we all agreed, a vision of consumerism gone wrong. But we couldn't think of a way round it. Clearly the supermarkets cannot sell everything they stock, but throwing out £3,000 worth of good food (when I totted it up, our haul came to £340) every night seems outrageous. Could they do more and throw away less? M&S doesn't have a reduced bin for food that's about to go out-of-date, said Cuthbert: 'They probably don't like the idea of tatty grannies rooting around in their stores.'

(I asked Marks & Spencer why they didn't have a reduced bin. 'Well, said their spokesperson Clare Wilkes, 'it's not right for us. It just wouldn't fit with M&S brand.' When food is going out of date, she said, the first option is to offer it at a discounted rate to staff. Other food is given to registered charities, notably the group FareShare. But meat, fish and eggs must, legally, be destroyed or made unfit for consumption, she said. Though clearly that had not happened at the M&S we'd visited.)

Pen finished one salad and started on another - an Orzo Pasta with Slow-Roasted Tomatoes. 'The funny thing is,' she said, 'you still feel guilty throwing away the food you can't eat - even though it's already been thrown away.' 'Yeah,' said Cuthbert, 'our eyes are often bigger than our stomachs.' 'But we've got more sensible over the months,' says Dave. 'We used to just gorge on all the bourgeois food. We'd grab packets of chocolate doughnuts. But that gets boring and now we're eating sensibly.' They're drinking too: a few weeks ago when the bins were full of unwanted grapes, Cuthbert used them to start making wine.

'What we're doing with this food is not an answer,' says Dave. 'But it is at least a gesture against the waste and excess.' 'And, besides,' said Cuthbert, 'we're skint. Some people might feel embarrassed at doing this - I'm not. I told my Mum and she asked me to get her some salad and bread. I'll get her a turkey for Christmas.'

Marks & Spencer's policy - if it is a policy - of leaving open access to the bins gets a thumbs-up from the freegans. At other supermarkets in this city they've found the food has had rotten milk, blue dye or even bleach poured over it. Sainsbury's compact all the waste food before binning it, while Tesco keep their bins locked until the rubbish trucks come. The freegans had only two complaints about Marks & Spencer. One is about the grotesque amount of packaging - virtually nothing is sold loose, it seems. The lunch I cooked did generate a fully stuffed carrier bag full of plastic and polythene. The other concerns all the packets of peeled and sliced fruit and veg that turn up in the bin - 'They do it for lazy lunchtime customers, and it's gone off by evening. It's very annoying.'

Dave and his friends have plans to spread the bounty. This autumn, they say, they are going to start up a local chapter of Food Not Bombs, the veteran American group that recycles unwanted food and gives it out free on the street and at public events. The work is, according to the Food Not Bombs website, a 'direct challenge to the injustice of the military/industrial economic system', though it's hard to see how handing out free food (vegetarian only) is going to bring down capitalism. Doing it outside Tesco might create more of a stir.

Freeganism arose in the States, out of radical anti-capitalist politics and the environment and animal welfare movements, during the late Eighties. It is more advanced in the States, where living outside the earn-and-spend economy and challenging the domination of supermarkets is, according to its practitioners, both a political philosophy and a lifestyle. In New York freeganism has extended far beyond dumpster diving - though that's the core act, the Holy Communion of all freegan believers. Freegan activities are extending to all walks of life: there are freegan legal services and freegan bicycle workshops in Brooklyn - where you get to mend or even build bikes for free. There are regular Really Really Free Markets in downtown Manhattan, which winningly advertise themselves as a way of 'fighting capitalism and boredom'.

Adam Weissman, a good-looking New York anarchist who is the Che Guevara of the freegan web networks and blogs, is famed for having bought virtually no consumer goods at all in 10 years: everything, from his computer equipment to his clothing, is 'discarded'. He lives in a squat. He doesn't need to work because he doesn't need money. One side-effect of 'minimising your participation in capitalism' is, as Weissman boasts, the chance to spend a lot more time with your family.

Weissman is involved in the influential freegan.info website, which publishes a Dumpster Directory for New York and 17 other US cities - it has entries like 'Broadway and 96th St. Gourmet Garage ... small, easy-to-unload dumpsters. On the night we went we found an abundance of fresh fettucini and ravioli in perfect condition' and a lot of heavy socio-philosophical material on anarchists and freeganism. Less political but more pervasive is freecycle.com, a fast-growing site where you can swap or give away just about anything. It claims 3.6 million members, largely in the States, but it now has a UK arm too. Deron Beal, who founded it in 2003, was interviewed in July by the business magazine Fortune, as another potential tycoon in the social networking web boom. 'It grew like bonkers from the get-go,' Beal says. 'You just needed a forum that made it cheaper and easier to give things away, rather than throw them away. You're making somebody's day and it feels good.' Beal has, of course, had offers from venture capitalists to turn Freecycle into a net-business. But that is not the point. 'The philosophy of the group is to let go of ownership.'

I did, in the end, warn the guests for the freegan lunch in advance. My friend Holger, who is a bit of a hippy, approved: he only wanted to remind me that he didn't eat meat. So he got a Haddock Mornay Meal for One (£2.99), complete with individual compartments of pre-made mash, peas and carrots. Vegetarians always eat worse. The children didn't mind about the bins - they asked if they could come next time. And they swallowed the tomatoey mince and new potatoes without more than average complaining. The only failure was my ice cream, made with M&S Loganberry Lakemead yoghurts (59p each) and lovely Tulameen raspberries (£3.99 for 220g): but then my ice cream always comes out like concrete. No one threw up.

My friend Bill, not a hippy but an Old Etonian, arrived at the lunch proudly clutching a present - a large slab of cheddar cheese, covered in a bloom of blue mould. 'I got it out of the neighbour's dustbin,' he said. 'Give me a knife and I'll scrape it clean. Don't you know that 40 per cent of the food we buy is thrown away? That's the real story.'

He's right of, course, that most food is thrown away not in supermarkets or warehouses, but from our own fridges and plates. Some 30 to 40 per cent of food is wasted in the UK, and that figure has risen by 15 per cent in the last decade. Government figures show that in 2005 about 17 million tonnes of food, worth up to £20bn, was put into landfill, even though approximately 25 per cent of it could have been eaten by people or animals, or turned into compost and energy. The figures aren't easy to disentangle, but certainly the consumer is as much to blame for the waste mountain as the food industry.

Discussing this late at night over the treasures gathered from M&S's bins, Dave said: 'The value of our waste is nothing. In Africa that skip would not stay untouched for more than a few minutes. I think food is too cheap.' Many would agree with him. Joanna Blythman says in her book Bad Food Britain, 'Cheap food is a national obsession'. The figures bear this out - nearly half of all shoppers put 'price' at the top of the list of factors that decide what food they buy - way ahead of 'quality' or 'taste'. Food is cheaper in real terms than ever before in modern history. Only 25 years ago we spent nearly a quarter of household income on food. Now that's down to 16 per cent, and falling. Four million people in the UK, we're told, cannot afford to buy the food they need: but the most remarkable thing about my freegan expedition was not the wealth on offer, but the fact that, in a city with pockets of serious poverty, there weren't queues down the road to get into the Marks & Spencer lucky dip.

Food is cheap, and that has made us lazy. With cheap food everywhere, in all seasons, we've lost our respect for it and its makers, according to Blythman, and so we're not fussed about throwing so much away. We're a nation that no longer does left-overs. My mother's fridge reminds me of this, with its rows of little bowls each crowned with a tatty piece of cling film; one has a dozen boiled peas, another half a roast potato and the third scraps of the joint of lamb that lasted my father and her nearly a week. In the door of the fridge one of the compartments is reserved for the heels of old cheeses that might come in useful.

Extreme? I have a friend whose mother keeps half a dozen chest freezers in an outhouse, all containing food and unwanted stuff from her vegetable garden she can't bear to throw away. That generation's waste-not want-not neuroses are understandable: they're born of painful memories of wartime shortage. What's stranger is how far we've shifted away from that mentality in just one generation. Is there no compromise between saving the carrot-tops for soup and chucking out one in every three of each ready-to-cook TV dinners you buy? Whatever happened to planning meals? Cheap food means we go to the shops now to buy the food we think we might like, not the food we know we need.

Freeganism won't solve all this, obviously. But it is, as the anarchists promise, a whole lot of fun. The biggest step is to dive that first wheelie bin, I was told: after that there's a whole new world out there, alive with the thrill of not paying. So far I've offered a free computer desk and a trampoline on uk.freecycle.org. As a newly committed urban forager, I've scouted the bins behind the very posh Italian deli round the corner from my office, ready for a diving expedition. If you want to join in, remember that Freeganism starts small and in the home - and most parents are already at it. But next time you polish off the remains of your kids' supper, think of it not as a sad modern-parent way to eat, but as a political act, a first blow in the battle against waste. As the freegans say: 'You are what you eat. So eat free!'

What our freegan brought home

Chocolate cake
5 bags potatoes
1 box oven fries
1 rice salad
3 ready meals
1 pasta salad
1 pack sweet peppers
1 box sushi
2 bundles asparagus
2 sandwiches
I pack chicken goujons
1 tuna salad
1 edamame bean salad
1 haddock mornay
1 double pack quiches
2 butterflied legs of lamb
1 pack fresh spaghetti
1 pot coleslaw
1 head celery
1 pack organic courgettes
1 bottle salad dressing
2 packs hot cross buns
2 sliced brown loaves
1 pack lime and coriander prawns
1 packet minced Aberdeen Angus beef
3 packs bagels
2 mozzarella and tomato breads
6 punnets cherries
2 punnets strawberries
6 punnets cherry tomatoes
1 whole roast chicken

Total Price £340

Waste not want not
How the supermarkets shape up

Supermarkets and waste

All the major UK supermarkets except Morrisons and Asda contribute their own-brand surplus food to FareShare, a charity that distributes it to homeless shelters and other such organisations. FareShare has six outlets in different parts of the UK. In London each week around 7-10 tonnes of food is redistributed to 80 organisations across London who provide meals to over 4,000 vulnerable people a week. FareShare can only accept food with at least 24 hours remaining before its use-by date. www.fareshare.org.uk

Tesco

Britain's largest supermarket chain sent 131,000 tonnes of waste to landfill in 2004 - it has not released any figures since. Spokesman David Nieberg told OFM, 'Our stock control systems help us keep food waste down to very low levels. This in turn helps us keep prices low - wasted food is of course an unnecessary cost and bad for the environment. While we do sometimes reduce food with a short shelf-life it's far less common now than it was only 10 years ago.' Tesco donates to FareShare, mainly from its depots rather than its stores.

Sainsbury's

Sainsbury's sent 91,000 tonnes to landfill in 2004. Since 1998 it has donated food to the Salvation Army, Food For All and FareShare, which it helped set up and has been working with for a number of years. In 2006 Sainsbury's donated over £3.3 million-worth of food weighing 6,300 tonnes. This was slightly less than the previous year because, according to a spokeswoman, the company has been addressing the root problems of food surpluses. Food that is beyond its use-by date is 'composted'.

Waitrose

Waitrose aims to minimise waste food by more accurately predicting customer demand. Food nearing its sell-by date, says spokeswoman Gill Rose, 'is often sold off to our partners' (as all employees are known). FareShare programmes are being rolled out to branches where there is a scheme in the area. 'We have opted for FareShare because the organisation has good systems in place to ensure that food reaches the right people.' Freegan foraging is not an issue, says Rose, because waste is kept in locked loading bays.

Marks & Spencer

M&S is a major contributor to FareShare, since all its food is own-brand. The company will consider approaches from any registered charity. Food nearing its sell-by date is also offered at a discount to employees, but an effort has been made to cut waste by merging sell-by and use-by dates where possible. By next year M&S hopes that all stores will be disposing of surplus food through anaerobic digestion schemes, that produce compost and gas that can be used for energy.

Is freeganism a solution to waste? Read more from Alex Renton and have your say about the article on our blog