In the plate-glass windows of the Schipperskwartier, one of Antwerp's four red-light districts, dozens of women sit waiting for clients. The choice is impressive - from blonde Baltic women charging about £30 for 10 minutes, to black women from Ghana and Zaire who are available for just £10.

As the night wears on and the number of prospective customers dwindles, market forces drive prices even lower. Every few paces you see tough-looking young men in leather jackets, sombre and watchful as they draw on their cigarettes.

These are the "watchers", whose job it is to protect the prostitutes from violent clients and to ensure that the proper fee is paid. But the watchers are also watching the women, making sure that they hand over their fee to the pimp and preventing them from running away.

Many of the prostitutes here are among the estimated 50,000 women who are brought illegally into the EU each year. Social workers in Antwerp report numerous cases of women who live as slaves, forced to work as prostitutes to pay massive debts to their traffickers.

"They are illegal residents, and the fact that they have been robbed of their money and identity documents deprives them of any hope of improving their situation. If they resist their traffickers and pimps, they face the threat of their families being told about their activities as prostitutes," according to Patsy Sorensen, a Belgian MEP who presented a report on the problem to the European Parliament last year.

Europol, the European police agency, believes most traffickers share the country of origin of their victims. The agency says that fewer than 5 per cent of victims have been kidnapped and most chose to enter the EU in the hope of improving their economic condition.

Although Europol is increasing the resources it devotes to tackling the sexual exploitation of immigrants, it acknowledges that the problem is likely to worsen over the next few years.

"The criminals adapt very quickly to measures taken by the law enforcement services and aggravate investigations. For example, the criminal sets up a ring of prostitutes in one member-state and then settles down or resides in a neighbouring country and limits their travels to the country where the criminal activities take place," says Europol. "To transfer their profits they are using reliable prostitutes or a `go-between' to act as couriers to collect and transport the money.

"The criminals open bank accounts in the names of their prostitutes, preferably in the country where the victims are coming from. All the earnings the prostitute makes are transferred to these accounts. In reality the bank accounts are controlled by the criminals," the agency says.

The European Commission called last week on all member-states to give a higher priority to combating trafficking in women and children. The Justice and Home Affairs Commissioner, Antonio Vitorino has proposed new EU-wide measures aimed at helping the police to tackle the problem.

They include common definitions of two criminal offences: trafficking in human beings for the purpose of labour exploitation and trafficking in human beings for the purpose of sexual exploitation. They also provide for the protection of victims during legal action against traffickers. The Commission will soon propose a measure that would offer temporary residence permits to victims willing to testify against traffickers.

"Because of their very nature, these problems recognise no national boundaries. They infect the whole of Europe and only a European solution can effectively combat them," Vitorino said.

The Social Affairs Commissioner, Anna Diamantopoulou, is launching an information campaign targeted at the trafficked women's countries of origin, and aimed at warning women about the fate that could await them. Many of the trafficked women come from states that are hoping to join the EU within the next few years and the Commission will put pressure on these candidate states' governments to co-operate with efforts to deal with the problem.

Most organisations dealing with refugees and illegal immigrants welcome the Commission's proposals to get tough with traffickers. But Lena Barrett, policy officer at the Brussels-based Jesuit Refugee Service, claims that one reason women resort to traffickers is that EU member-states have closed off most other routes.

"There are certain barriers in place that don't have to be in place," she says. "There is no mechanism for people to go to an embassy in their own country and seek a visa. And it is important to remember that being smuggled is no reflection on the strength of someone's claim to asylum."