Girls' club that's a refuge for boys who refuse to grow up

Third rule of the Groucho Club: "The ingestion into the bloodstream of powders, pastilles, potions, herbs, compounds, pills, tablets, capsules, tonics, cordials, tinctures, inhalations, or mixtures that have been scheduled by Her Majesty's Government to be Illegal Substances of whatever class is firmly prohibited by Club Rules, whether they be internalised orally, rectally, intravenously, intranasally or by any means whatsoever. So let it be known."

Such elegantly worded and comprehensive regulations have not prevented the odd moment of excess during the history of Soho's most famous private club. Here it was that broadcaster Rowland Rivron, steaming on Pisco Sours, rode down the stairs on a waiter's mountain bike, crashing into the bar area and ending up covered in plaster as well as plastered; where Jonathan Ross's agent Addison Cresswell, the worse for wear, set fire to the building by falling asleep with a cigar in his mouth; and where Damien Hirst ignited the chest hair of the publicist Mark Borkowski (requiring him to seek hospital treatment). It was also where Toby Young seduced a Princess Diana impersonator in the toilets. All right, Young was barred for that, but you get the picture.

The Groucho is a place of such legend within British media circles that it seems to have been around a lot longer than the 25 years it will have chalked up this summer. Where else might you find Bono on the piano playing "Happy Birthday Mr President" for Bill Clinton, or Moby tinkling those same ivories with accompanying vocals from members of The Clash, New Order and Coldplay?

Fifth rule of the Groucho: "The wearing of String Vests is fully unacceptable and wholly proscribed by Club Rules. There is enough distress in the world already." You may have guessed by now that these rules were compiled by Stephen Fry, a founder of the Groucho, which has a membership of 4,000, all of whom must be proposed by two existing members and then approved by the committee.

Some of the above might make the Groucho Club seem like a refuge for boys that refuse to grow up. Margaret Levin, the club's managing director, sits in the restaurant area as her impish front-of-house Bernie Katz (also known as The Prince of Soho, and said to be "the custodian of a thousand celebrity secrets") explains why this haunt is so appealing to women. "The Groucho Club was opened at the behest of strong ladies in the publishing world," he says, referring to Virago's Carmen Callil and Bloomsbury's Liz Calder. "It was a time when ladies couldn't behave in the same way as men in doing business. That was the prime reason for opening up The Groucho Club." Even now, a majority of members are female, among them Tracey Emin and Nigella Lawson. "It's 60-40 in favour of women, which is lovely because it has served its purpose," says Katz.

There is a lot of competition these days among London clubs aimed at the creative crowd. Just round the corner there's Nick Jones's Soho House, then there's The Hospital Club run by Will Turner in Covent Garden, and a raft of other newly opened rivals. But Levin says the membership criteria will remain a "rigorous process". The club committee, the names of which she refuses to reveal, "is made up of high-powered people who sit at the top of their tree and between them they will nearly always know the applicant and have quite a strong view of them".

The Groucho has reciprocal arrangements with international clubs including the Rapongi Hills in Tokyo, the Norwood in New York, the Spoke in Toronto and De Kring in Amsterdam. Unlike the expansionist Soho House, the Groucho has no immediate plans to open branches elsewhere in Britain, though Manchester's growth as a media hub is acknowledged.

What it will do to celebrate its 25th birthday is launch an annual financial award for "up and coming media creatives". Levin says: "I'm hoping it can grow into something like the other big awards, I feel quite passionately about it."

The Groucho Club has also marked its anniversary by upgrading its famously grubby bedrooms. "We've cleaned up our act. Jesus! Did you ever see them? They were desperately in need of doing," the managing director admits. "Were they a running joke? Yes, that's the nice way of putting it."

Despite the Groucho's enduring reputation for naughtiness, there's still a certain decorum to the Dean Street haunt, which was once an Italian restaurant called Gennaro's. There there might be iPod dockers and flat-screen televisions in the newly refurbished rooms, but according to Fry's second rule of the Groucho: "The use within the Club of Mobile, Cellular, Portable or Microwave-Controlled Telecommunication Instruments is an anathema, a curse, a horror, a dread and deep unpleasantness and shall be prohibited in all locations." There speaks the outside world's undisputed King of Twitter, and his 4,000 Groucho pals.