The Danish, Swedish and Finnish deputies mostly come from the Socialist, Liberal and Green factions in the parliament and include former Danish Prime Minister and the current president of the European Party of Socialists, Poul Nyrup Rasmussen.

Two conservative MEPs, Sweden's Charlotte Cederschiold and Finnish deputy Eija-Riitta Korhola, also joined the initiative.

The Copenhagen-based Nordic Council was founded in 1952 as a forum for Nordic parliamentary co-operation. It adopted the measure on prostitution and hotels in 2006.

Prostitution is legal in Denmark and Finland. It is also legal to sell sex in Sweden but against the law to buy it.

"The background for this decision is of course the growing understanding of the harm that prostitution causes for the people involved in prostitution as well as the role of prostitution in organised crime," the Nordic MEPs said, noting the "praxis of many hotels as providers of sex trade with humans, as if they were dealing with an ordinary commodity."

There is plenty of anecdotal material that some well-paid European Parliament workers and MEPs, away from their partners for the week, pay for sex during the monthly plenary session in Strasbourg. But evidence is scarce.

Hotels deny they would ever help a guest find a prostitute, while some smaller guest-houses exclude prostitutes who normally live with them in order not to put off EU clientele during the plenary sessions.

"We don't have a demand for such services. If clients don't demand a service we don't offer it. It's possible it may happen. But it doesn't happen here," the manager of one large Strasbourg hotel told EUobserver. "If a guest brought back somebody, they would be very discreet."

The Strasbourg office of French anti-sex trade NGO Mouvement du Nid said the European Parliament has no impact on levels of street prostitution.

"The parliamentarians are not interested in street prostitutes. They prefer escort girls, call girls of a slightly higher level. They find little adverts and make telephone calls. That's how they take care of business," the NGO's Isabelle Collot said.

"This is very hard to measure. It's not public - prostitution is illegal in France. But if [the MEPs] have written this letter, it means the problem must exist. It's a symbolic gesture but it could start a useful debate."

"Not all men are clients," Ms Collot added. "I hope there are still some faithful [European] deputies, for the sake of their wives."