Keeping the jetset amused is not an easy business. In the ongoing pursuit of decadent destinations in which to be extravagant, shocking and scantily clad, the beautiful people quickly tire of Bacchanalian furores and all-night party concepts. Spare us the yachts, they plead. Deliver us from Klosters at New Year, the renovated mountain-top Ibizan fincas with dedicated disco rooms, the pseudo opium dens of the once again debauched Berlin.

And yet, for nearly four years, one jetset servicing scene has held its appeal. The four incarnations of Nikki Beach at Miami, St Tropez, Marbella and the Caribbean island of St Barts are 'the sexiest places on this earth', according to one prominent member of the idle rich who I encountered propping up the sushi bar in St Tropez. 'It's the best party in South Beach,' says Jack Donohue, celebrity model scout and elder statesman of the Miami scene. 'It's where the most beautiful women hang out,' adds the concierge at Byblos, St Tropez's most fashionable hotel, residence secondaire for the Nikki Beach regulars.

Stripped down to its component parts, Nikki Beach might not seem like a big deal. The empire is focused around four bars with restaurants, a DJ booth, and elaborate sun loungers ranged around split-level decking. Each one is located in self-contained areas on four of the most celebrated beaches in the world. They aren't resorts, precisely. There isn't a Nikki Beach hotel or a spa. And they close at midnight.

They're uniformly white; the carved crocodile benches are padded with full white pillows; the tepee huts into which the clients disappear (officially to change into their Missoni swimwear, unofficially to indulge whatever extravagant whim has just taken their fancy) are white; so are the vast square, decking-supported beds that surround the pools in St Tropez, the hammocks slung between palms in Miami, the raised VVIP-only curtained tikki huts, the awnings.

The staff are hired for their outrageous beauty, glamour, charm, and absolute belief in the Nikki Beach concept, all of which (give or take the company loyalty bit) makes them splendid company. In the day they are clad in white, too: T-shirts emblazoned with the Nikki Beach logo on the front, and the legend 'Tell Only Your Best Friends' on the back; diaphanous drawstring pants; bare feet. At night they wear the same in black, 'mostly,' one barman tells me, 'because sooner or later we'll probably end up in the water, and if we're wearing white, and no underwear, well, you can imagine.'

Indeed. But beyond that, it's hard to identify what makes Nikki Beach so irresistible to a certain, affluent, sun-chasing, party-addled set who tour from one location to the next as the sun shifts, leaving Nikki Beach St Tropez for Nikki Beach Marbella, and Nikki Beach Marbella for Nikki Beach Miami. The only way you can understand the appeal is to experience it. Of all the Nikki Beaches, St Barts is the smallest, and - partly as a result of this, partly because no one but the truly loaded makes a day-trip to St Barts - also the most exclusive. It's barely been open a year, yet already talk of outrageous activities is rampant among the international party crowd: rumours of champagne showers; of celebrities table dancing, semi-naked; of goings-on in the relative privacy of the tiki huts.

Nikki Beach St Barts is next door to the tiny Aeroport St Jean. If you fly with Francis, the cool, Indiana Jones-esque pilot of one of the battered eight-seater planes that transfers visitors daily from Antigua to St Barts, he'll point it out from the air. 'Oooh, Nikki Beach,' he'll say, sparkling knowingly behind his huge gold-framed Aviator sunglasses. 'Party!' Then he adds: 'You want to know who the most famous client I had was? Diana.' She wasn't headed for Nikki Beach since it didn't exist at the time - but you get the idea.

Current jetset etiquette dictates that on arriving on the island, the Nikki Beach crowd retire to one of three hotels: Carl Gustaf, Eden Rock or the divine oasis of chic, Isle De France. Unless of course, like Mariah Carey, you're staying in a super-luxe villa hired through Sibarth, the It estate agent in St Barts' biggest town, Gustavia. Or on a yacht. Or at actress/professional tantrum-thrower Shannen Doherty's private holiday residence.

Once there, you shower and change, ideally into a Pucci kaftan or Bellacosa crochet shorts, and flip-flops, which this year should be decorated with shells (regardless of your gender), and have (crucially) a very thin sole. Now you're ready, but if it's a moment before 2pm you shouldn't head off yet. Of course, for 'un vrai jetset', the hangover from the previous night's scandalous activities will ensure this kind of faux pas is not a possibility.

Nikki Beach doesn't look like much from the outside. A discreet entrance marked by a flag bearing the Nikki logo is squeezed between a couple of stores peddling designer beach wear. You don't pay for entry - you just wander in, down some decking steps, and you're there, in the white, sensual, palm-fringed, ocean-licked heat and heart of it all. Just as you're being knocked sideways by the extravagance of the venue, you get knocked even further sideways by Alex, a lithe twentysomething of considerable gorgeousness, who is somehow unassuming enough not to intimidate the pants off all incoming guests. Alex could have been tailormade for Nikki Beach's purposes. A Swedish model with a 10-year career behind her, she threw it all in when she came to the island on a shoot five years ago and fell in love - both with St Barts and with Jacques Dumas, now her husband and also, she points out proudly, the resident Nikki Beach DJ.

Alex, like Patrick, the general manager of Nikki Beach St Barts, and Karen, head bargirl, is full of tales of champagne-spraying and Nikki Beach hedonism. 'The person who can order the most champagne and spray it all over the club, that's the competition,' she says. 'That's what they all love to do. It's great! It's fun!' Champagne, for the record, costs over £400 a bottle here.

I visit in November, relatively early in the St Barts season, and the club has only been open for a fortnight this time around. There isn't enormous evidence of the international jeunesse dorÀe quite yet. They are a fickle bunch, and a few days of dubious weather has apparently convinced many to change plans at the last minute and fly out to Miami instead. Yet those who are here, lounging about, are instantly identifiable as Classically Nikki. The Nikki Beach crowd are a pioneering bunch in jetset terms. Hipper than the Eurotrash, younger and less preppy than the Hamptonites, infinitely more luxe than the Ibiza and Thailand-frequenting pseudo-hippy trustafarians, they're the latest incarnation of the idle rich. Think Paris and Nicky Hilton, P Diddy and Ivanka Trump, much cooler, less coiffed daughter of Donald and Ivana.

This crowd have their own traditions, culture and look. They're mostly American (around 90 per cent, Patrick estimates) but have modern European sensibilities. Their tans are light, golden and luminous instead of the terrifying mahogany of the old jetset. Their hair is Caribbean-tangled and sun lightened on account of the enormous amount of time they spend launching themselves off the side of speedboats and into the sea. Their clothes are cool, discreetly labelled and probably a bit travel-rumpled.

They're an oddly democratic bunch. Regardless of how you came by your money, providing you have it in bucketloads, you are embraced by the Nikki Beach set. Nobody looks down on the A-list celebrities who visit Nikki Beach in droves - although, equally, nobody offers them star-struck reverence either. 'That's why the stars love it here,' says Eric Omores, co-owner and architect of the Nikki vision, sipping on his usual combination of mineral water and a non-alcoholic crÀme de menthe while rearranging his black embroidered kaftan and lounging back against a wooden croc bench. 'They can come and just be with people.'

'BeyoncÀ and Jay-Z came to see us in St Barts with no reservation and no entourage around them,' says Patrick. 'They had a little table in the corner. Two bottles of Cristal. And then the DJ starts to play Moroccan music, and they call everyone over: come and dance with us!'

'And Puff Daddy,' adds Alex. 'He loves to mix, so he comes up, pushes the DJ out and starts to play. And Mariah Carey, she requests songs from the DJ just like anyone. Buys champagne for everyone.'

Senegal-born, educated in France and trained in event production, Eric Omores has a long history of running bars and clubs in every jetset location in the world. In 1993, in conjunction with Simply Red's Mick Hucknall, he opened Bash, a phenomenally successful night club in Miami. Then he teamed up with Jack Penrod, an established name on the South Beach club and restaurant scene, and started revamping Jack's flagging Penrod's Beach Club. 'We didn't really invent anything,' explains Eric. 'But from a business perspective, there was a niche. There was nowhere that had the feel of the European beach club, like Voile Rouge in St Tropez. Nowhere that had the ambience, the sophistication, the food, the music, the decadence. Somewhere to party hard but also take care of yourself, to relax, eat well, hear good music. This is what I wanted. I wanted a lifestyle.'

This, Eric decreed, was as much about the food ('gastronomique'), the music ('laid back, world music, down-tempo, very European') and the fixtures and fittings ('European again, but minimalist, contemporary') as it was the high-octane partying. They christened the fully developed concept Nikki, in honour of Jack's 20-year old daughter, who was killed by a drunk driver. Eric invited every key name on the glittering contacts list he had built up through 15 years on the high-end party scene, and the club launched in winter 1999 to wild acclaim and, crucially, serious endorsement by the model fraternity.

'People came for the hot girls wearing next to nothing,' explained Tommy 'Pooch' Puccio, former co-promoter of Nikki Beach. 'From the start we decided to make this a sexy place.'

This transformed the Miami site into something more than a club. Within a year it had become a fledgling brand. Le jetset wanted to be associated with it every bit as much as Nikki Beach wanted them. The staff T-shirts became cult fashion items. The ashtrays started being stolen.

And so Eric and Jack, and Jack's wife Lucia, began work on brand-extension exercises: a record label which produces Nikki soundtrack CDs; a clothing range; a magazine. And, finally, they began plotting to launch on other beaches. St Barts opened at New Year 2002/03, St Tropez in spring 2003 and Marbella halfway through last summer. They were considering Bali when the bomb went off, so that decision is still on hold, and Dubai is a future possibility. Eric has a smooth line on integrating Nikki Beach into new communities. 'These markets we come into - it's us that adapt,' he insists. 'We don't want to do something against the grain.'

Patrick, the hyper, wiry, super-French general manager, is less diplomatic, at least about the bar and restaurant's rivals on this island. ' On St Barts, they hear about us, this dream team, and everyone hated us when we first arrived. But we came anyway. And we worked. Now they don't do as well as us, so we don't care. They hate us, we don't care. We just party. Every time.'

And the world's richest and most decadent continue to party with them. Sunday night is fashion show night. The staff model pieces from the Nikki Beach clothing range and le jetset show their appreciation by whooping and spraying the models with champagne. Tuesday afternoon: American Elle arrive for a fashion shoot. Le jetset are drafted in to frolic in the back ground. Wednesday night: Caribbean barbecue night. When she's not gliding between tables and charming the Prada off her guests, Alex talks happily about New Year's Eve, biggest night in the St Barts calendar, when prices are increased tenfold and Nikki Beach sells more champagne than in the rest of the season put together.

This much is clear. The Nikki Beach party is a long way from ending. No one's about to turn the lights on, start collecting discarded Cristal magnums and ask the international party crowd whether they've got homes to go to. Eric has ideas. He talks about big, Nikki Beach branded resorts. Hotels and spas. Marinas. You get the feeling they aren't far off because Eric isn't given to aimless dreaming. 'We just want to take care of people,' he says. ' Make them feel great. That's what we're for. That's our philosophy.'

'And we've never thrown anyone out,' adds Patrick. Never? Even when they've gone completely crazy?

'Too crazy for Nikki Beach?' he asks. 'I can't imagine what that would be.'

· A seven-night stay at Isle de France with Caribtours (020 7751 0660; www.carib tours.co.uk) costs from £2,160 per person based on two sharing. This includes return flights to St Barts, transfers to and from the hotel and accommodation with breakfast each morning.