DO NOT go on holiday with your friends this year. There is every chance that they will not be your friends when you get back. Unlike divorce or crime figures, there are no statistics available to chart the number of blissful friendships shattered by the shared holiday. But it remains a popular and enduring formula, even though the annual two- week bloodletting frenzy indulged in by the British middle classes makes seal-clubbing look like an act of affection.

Of course it all makes such good sense back in Hampstead or Frogmorton. There is that easy-going couple Harry and Polly, such a hoot at dinner parties; wouldn't it be terrific if they came along with sensitive Sylvia and Martin and their three sweet children and we all rented a gite for a fortnight in the Dordogne? Think of the savings, we could share the driving, and they all simply adore our kids, don't they?

Meanwhile, in the polite sharing of sunbeds and sun-block, no one suspects the Grand Guignol to come. It may be fun for a couple of days, until folk begin to relax. That is when the temperature begins to rise, and those carefully cultivated orchid-like relationships begin to wilt, or to stink and seep poisonous resin.

Careful study shows that the three most common areas of dissent likely to sour a shared holiday are: Other People's Kids; Other People's Money; and Other People's Toilet Habits. In the course of researching my most recent novel, in which three families share a villa in the Perigord, I was astonished at the torrent of spleen and invective manifested whenever I invited anyone to talk about a communal holiday that had gone to hell. Kids for example.

"They had the run of the house; they never stopped squabbling; never washed a single cup or plate. They were rude, dirty, loud and they broke things. And as for their three children..." This is Polly talking about Sylvia and Martin and their brats. "Their baby contracted chicken pox and cried constantly from day one: a banshee-like wailing, morning, noon and night, so that Harry and I literally had to wear ear-plugs to supper for the last five days."

Then there is the issue of Money, usually concerning the way other people are not spending it. "The monster actually harangued us at the breakfast table, adults and children alike, giving us a dressing-down..." said Martin, a teacher, talking about Harry, a computer programmer. Harry, it turned out, was one of those skinflint Biro-wielding individuals prepared to scribble complex algebraic equations on a restaurant tablecloth in order to apportion a bill precisely according to who ate what. "Just because we'd gone through two bottles of ketchup in the first week. After that he wanted to know how we could possibly have exhausted a toilet roll multi- pack in seven days."

Stinginess on the one hand, and profligacy with the communal resources on the other, is capable of tilting reasonable individuals into paroxysms of loathing. The third category, however, is less easy to account for, relating as it does to the extreme prejudices and caustic hatreds generated, quite irrationally, by idiosyncratic behaviour. Certainly, it is possible to overlook the foibles of human nature until they glint at you on a daily basis.

Shared bathrooms can present more than the usual queuing problems. One woman pushed open a door with a faulty catch to be confronted with other than the usual embarrassment. One of her holiday companions was in the process of injected himself, and not with insulin. It can come as a surprise to discover a friend of five years' standing is a smack-head. A lot of talking had to follow to salvage the holiday. It is entirely possible you may not have been invited purely for your bonhomie and your sparkling conversation. You may find yourself trying to shore up other people's disintegrating relationships. "We were clearly there to provide a demilitarised zone," said one man who witnessed the trajectory of an impending divorce from terminal to murderous. "They'd hoped they'd be more civilised with other people around them. It didn't work. One day there was actually blood in the swimming pool."

It does not always have to go wrong. I went on a shared holiday while writing my novel and made no secret of the fact that I was on the look- out for material: some revealing group-dynamics, an insight into our lowest instincts.

Unfortunately, everyone behaved impeccably. It was hugely disappointing. Perhaps it was the notion of someone armed with a notebook that prevented the company from relaxing into their true, brutish selves. But then again, not every author is the amiable and forgiving, companionable holiday chum that I am.

A well-known writer of thrillers recently shared a villa in Tuscany with his editor, the editor's wife and a few other cronies from the publishing world. Being a creature of extreme habit, the author insisted on a cup of hot chocolate and two digestive biscuits at 10.30pm every night before retiring to bed. Everyone else was given to noisy carousing until the early hours, but the author got his own back every morning by leaping in the pool at 6.30am with a thunderous splash and an unnecessarily virile war cry as his body hit the water.

Finally, one night, the editor's wife spitefully hid the digestive biscuits. Everyone thought it a terrific prank until the author's frustration turned to tantrums, and finally to rage. Things went too far to admit who was responsible until the editor nobly admitted he had hidden the wretched biscuits as a joke. The bestselling author returned home and instructed his agent to switch publishers.

Proving that holidays and friendship, like business and friendship, and indeed like business and holidays, simply do not mix. Which is why hundreds of British couples return from their precious fortnight inwardly seething and muttering darkly about never making the same mistake again. Only the following year, during their somewhat lonely packaged alternative, to look back with nostalgia on the hell that is other people, and plans to do it again. Perhaps it is because we have got forgiving natures.

Or perhaps it is the advantage of hindsight that sometimes makes you realise that, when talking about the monster you went on holiday with, the monster is you.

`The Storm Watcher' by Graham Joyce, Penguin, pounds 5.99