Carl, a sauna-pink Swede with the shoulders of an ice-hockey player, has just made the 15-minute journey across the Öresund bridge to buy sex in Denmark.

“I can feel free here,” he says, stretching his arms out wide at the bar of the Spunk Club in central Copenhagen on a Saturday night. “I can breathe.”

The freedom to buy a prostitute does not figure in any charter of human rights. Quite the contrary. In Sweden paying for sex is a crime punishable with a possible six-month jail sentence or a hefty income-linked fine. Perhaps the worst penalty for errant Swedish males is the official court summons addressed to the family home; an embarrassment that has ruptured many marriages.

In Denmark, by contrast, prostitution has been decriminalised. Nigerian and Romanian women competed for Carl’s attentions when he staggered out of the Spunk Club, while a brothel next door bore a sign saying: “Here Only Danish Girls.”

The builders of the Öresund bridge linking Malmö with Copenhagen did not anticipate that it would become a runway for transpontine sleaze. Stretching 8km (5 miles), linked in with tunnels, it is an engineering triumph. But, like the Channel Tunnel, it has also brought two dissimilar and often competing societies into an uneasy proximity.

For the most part it is harmless: Danes buy homes in Malmö because property prices are so much lower and commute to work over the bridge. Bringing two Scandinavian societies closer, however, pits one welfare state mentality against another. Denmark, proud of its tolerant traditions, has allowed the hippy colony of Christiania to flourish in the heart of Copenhagen since the 1970s.

Now Swedish teenagers are taking taxis over the bridge, stopping off at the settlement, stocking up on marijuana, and driving back home. The Swedes are irritated; the Danes sensitive - police occasionally raid Christiania’s Pusher Street to show that they have not lost control – but ultimately they are not that bothered.

The true flashpoint is prostitution. Nothing better highlights how the model Scandinavian societies are now at odds over the correct road to Utopia. The Swedish law, punishing clients but keeping prostitution legal, is based on the premise that prostitution is a form of violence against women. “We don’t have a problem with prostitutes,” says Kajsa Wahlberg, of the human trafficking unit of the Swedish police. “We have a problem with men who buy sex.” Inspector Wahlberg estimates that the number of prostitutes in Sweden fell from 2,500 in 1998 to 1,500 in 2003, and the trend is still downwards.

But there are two problems with this law, which may be adopted by Norway and which is widely admired by feminist and socialist MPs across Europe, including Britain. The first is that it has taken Sweden even closer to the Big Brother state. Customers are secretly filmed going in and out of brothels. The police then confront them with the evidence. Phone tapping landed a senior judge in trouble with the police after he contacted a young male prostitute; the judge - who had always given mild verdicts in prostitution cases - resigned.

“Danish lawyers wouldn’t be able to do anything with this kind of evidence even if we had Swedish-like laws,” says Ulrik Dahlin, who has been following the ups and downs of the Scandinavian prostitution debate for the Informationen daily newspaper.

“But the main problem is that prostitution has gone underground in Sweden,” he says. “I watched how kerb-crawlers now behave in Sweden – the client draws up, agrees another rendezvous with the woman and drives on rather than risk the police filming the woman as she gets into his vehicle.”

Thanks to the bridge across the Öresund Strait frustrated clients such as Carl can now travel to Denmark, despite the toll charge of €30 (£24). The bridge was opened the year after the Swedish law came into effect. Since then the number of prostitutes in Copenhagen has doubled to 6,000.

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