Charles Astwood is the kitchen nightmare that Gordon Ramsay can't fix. More horrifying to many restaurateurs than AA Gill, Fay Maschler and the man from Michelin all coming to dinner at once, Astwood spends at least part of each day listening to London's top chefs scream at him. And why? Because Charles Astwood has transformed ordinary punters into critics.

Astwood is the founder of London-eating.co.uk, a website dedicated to restaurant reviews written by ordinary diners. Did you eat the best langoustine in London last night? Or was the service snooty, the prices outrageous and the food mediocre? Four star or no star, gushing or gruesome, type it up, and so long as you don't libel anyone, Astwood will publish it, along with all the other customer reviews and ratings for that restaurant.

Since its launch in 2000, the site has grown to become the largest online restaurant guide in the UK, with more than 650,000 unique users visiting the site each month and more than 100,000 reviews, all submitted by ordinary diners (its sister site, City-eating.com, covers other UK cities such as Manchester, Edinburgh and Brighton, as well as foreign cities, but hosts far less reviews). The appeal is obvious - reviews are honest (brutally so, in some cases) and up-to-date. (The fact that its reviews invariably appear first when you Google a restaurant's name must also drive diners to the site, too.) It's like a word-of-mouth thumbs up or down for nearly every leading restaurant in London, and Astwood is evangelical about it. Fine dining is no longer the preserve of the very rich, but a product like any other, he says. After all, if you buy a DVD player on Amazon, you'll certainly look at the customer reviews, so why not do the same for a romantic dinner for two at, say, the Ivy?

Unsurprisingly, it's making restaurateurs feel vulnerable. Jamie Oliver, after receiving sustained criticism on the site of his restaurant, Fifteen, felt he had to respond to the online reviewers. "I don't know why I'm bothering reviewing your comments at midnight," Oliver wrote on the site in January 2003, "but as this project has been a real love and dream of mine all my life, I feel I should post something to make myself feel better ... We are getting there and the food is really getting close to where I would like it to be. Thank you for your comments, I have found them very interesting and will act on all your points as I am every day."

Michel Roux Jr, chef de cuisine at Le Gavroche, the first British restaurant to be awarded three Michelin stars, says that he knows how Oliver feels, but isn't sure that it's the way forward. "I think these sites are good fun, but we probably shouldn't take them too seriously," he says. "After all, to review Le Gavroche you need to have eaten at the other top restaurants in London. Otherwise, how can you compare?" As most of the top 10 restaurants in London have a price tag reaching £100 a head and rising, the pool of qualified reviewers by this measure is necessarily small.

Eddie Hart, owner of Fino, a tapas bar in Bloomsbury, has recently started taking bookings online via the site, but doubts that his customers rush back to the office after a good lunch to type up a review. "There are nine reviews for Fino on the london-eating site, but we've had 28,000 customers through the door," he says. "I like the idea of Joe Bloggs telling us about his experience, but I'm just not sure our kind of customer uses these sites." Rose Gray, co-founder of the River Cafe, agrees. "Sometimes you read the reviews and it's hard to believe that the writer has even been to the River Cafe," she says. "At least with a food critic you know that they actually know about food."

The idea that "our sort of customer" would never be so vulgar as to review a restaurant online is a common one, but Astwood has bad news for Hart and Gray. His users are resolutely ABC1 types - user surveys reveal that, on average, they eat out at least twice a week, one fifth earn more than £50,000 and 90% type up their entries during office hours. "We've got several regular reviewers who eat at Michelin-starred establishments two or three times a week. But, in a way, that's not really the point," Astwood says. "Everyone's paying money to eat there, and at the end of the day their opinion is just as valid as a professional critic's."

But what about the restaurant reviewers? Jay Rayner, food critic for the Observer, thinks that he and his colleagues will be stuffing their faces on behalf of the dining public for some years to come, but says that the power of the web should not be underestimated. "I'm only one voice, and my opinion ends up in the cat-litter tray after a week. Reviews on the web hang around for ever; and for a small restaurant it can cause serious problems."

Hannah Tyekiff, marketing manager at No 5 Cavendish Square, is one of those who has asked Astwood to remove reviews. "Customer websites only give you one side of the story," she says. "Bad reviews often come from customers who were incredibly drunk, or very rude to our staff." Tyekiff and her team examine all comments, but she still thinks that the system is unfair. "We have stacks of letters congratulating us on our restaurant," Tyekiff says, "but if someone sees a bad review online we lose a customer before they even walk through the door."

Claudio Verrilli, manager of the Drawing Room in Lavender Hill, says that he checks all the online reviews, but if your restaurant gets targeted it can be lethal. Verrilli says that London-eating.co.uk removed a number of reviews of the Drawing Room because an investigation demonstrated them to have been faked. "There's a lot of competition in our area, and most people are struggling," he says. "Some people think that writing bad reviews about other restaurants is a good way to improve their chances."

Astwood's team moderate every review, checking for plausibility and accuracy, but he admits that restaurants will always try to skew the reviews. "I'm confident that 99.9% of reviews are genuine. I want to provide the best impartial assessment of what a restaurant is really like for ordinary paying customers," he says. "If we allow the site to be abused, we might as well give up."

Astwood argues that his system is, in fact, fairer to restaurants: rather than getting one chance to impress a spoilt, over-fed critic, restaurateurs get as many bites of the cherry as they can inspire diners to write. In 1987, on the cusp of recession, Grace Ilic set up Le Mercury in Islington, with a mission to serve the very best food at the lowest possible price. Eighteen years later, Le Mercury features in London-eating.co.uk's list of the top 10-rated restaurants in London (see box), in the very fine company of, among others, Gordon Ramsay (three Ramsay-inspired restaurants make the list). Online reviews have raised its profile far beyond what it could achieve through word-of-mouth alone.

"The reviews are amazing - I'm shocked," says Ilic. "The internet means that we are still being reviewed. We would not normally be reviewed in the papers because we have been here 18 years."

Astwood believes that his site also provides a service to restaurateurs. "I know for a fact that the Ivy checks our site to see what is being said about them, and they are not the only ones," he says. "It's a much more effective customer-interaction tool than trying to persuade people to fill in a comment card." But Rose Gray is unimpressed by this argument. "If we have an unhappy table we will know there and then," she says. "We will deal with it on the spot."

For some, though, nothing can replace the expertise of the professional restaurant critic. Egon Ronay has been reviewing restaurants since 1957. He is astounded to hear that ordinary restaurant-goers are posting reviews on the web. "It is of no greater use than someone venturing an opinion as you wait in the queue at the supermarket," he declares. "Or somebody saying that they had a very good dinner, while you are both sitting in the doctor's surgery."

Top 10 London restuarants as rated, in order, by www.london-eating.co.uk reviewers:

· Mosaica @ the Lock (Tottenham)

· Gordon Ramsay At 68 Royal Hospital Road (Chelsea)

· Michael Moore (Marylebone)

· Lightship Ten (Tower Bridge)

· Terra (Fitzrovia)

· Le Mercury (Islington)

· Chez Bruce (Wandsworth Common)

· Gordon Ramsay at Claridge's (Mayfair)

· Petrus (Knightsbridge)

· 1880 at The Bentley Kempinski (South Kensington)