THEY arrived at Newcastle Airport on a flight from Paris on Easter Sunday. Clutching false Japanese passports, Mei Fang Weng, 15, Xiu Ming Lin and Yun Jen He, both 16, made their way to passport control, only to be stopped by immigration officials. The three nervous schoolgirls announced that they wanted to claim asylum and were duly put into the care of social services and housed at Elswick Lodge, in Newcastle. Three days later, they vanished.

No-one knows exactly what has happened to the teenagers, but it is feared that they may have been sucked into the seedy world of the cheap sex trade - one of the fastest growing international crimes.

Charity workers and police describe its victims as sex slaves. The traffickers target the poorest, most vulnerable people across the world, luring their parents by thoughts of better lives in the West for their children and by promises that a working daughter can send money home. Others are simply abducted.

Says Juliet Singer, head of police liaison with the National Missing Persons charity: "They pick on a tier of society that is most vulnerable and somebody offering money for their beautiful daughter will be welcomed with open arms.

"Some girls, particularly the Eastern Europeans, believe that they're going into high class prostitution or to work in high-class restaurants, essentially that they're going to be looked after. Others don't have any idea what they're getting into at all."

National Missing Persons first became concerned about people trafficking in the mid-1990s when they began to hear of West African women who were being brought into Britain and forced into prostitution. Today, around 80 per cent of the 8,000 prostitutes working in London's brothels, saunas and massage parlours are foreign nationals, mainly from eastern Europe and south-east Asia.

"They are usually from war-torn countries or countries which have been hit by famine or some disaster," says Juliet.

"As soon as the tsunami happened the traffickers had moved in and rounded up a lot of the youngsters under the age of 16. That was in the first week.

"The route here can be varied. They will use smaller airports such as Newcastle as we have seen, Manchester, Belfast or Cardiff. The ports are also vulnerable. Plus they can even bring them in via a small boat. About three years ago, 20 Ugandan women turned up at Colwyn Bay in a yacht."

Once in Britain, the women will phone their contact - the trafficker or pimp - who will give them instructions on where to go (it's believed the Chinese girls in Newcastle left with a mystery man). Then they are set to work. Many have their passports and travel documents taken from them so they cannot flee. They are frequently forced to live in squalid conditions and can be sold between brothels for up to £10,000.

The attraction of the foreign prostitutes to the punters is that for as little as £30, they can have unprotected, often rough sex in contrast to the days when unprotected sex used to cost considerably more. There is also the lure of young flesh - some of the girls being smuggled into the country are as young as 12 years old.

Raped, beaten and threatened on a daily basis, the women find themselves working 24 hours a day, having sex with up to 25 men. According to Juliet, an average 16-year-old can earn around £300,000 to £400,000 a year for her pimp. But it is rare that the women will get to see any of the money. Rather, it will be given to the pimp to pay off the "debt bondage" of thousands of pounds charged to bring her to Britain. The debt is rarely paid off because the pimp will add on food, rent, condoms etc.

Those who do manage to flee are often riddled with sexual diseases such as HIV, syphilis or chlamydia and have nowhere to go. The Poppy Project in London offers practical help and counselling and has 25 residential places. But it has been inundated with referrals since it began in March 2003 and by February this year, had already had 250.

For those who find themselves trapped in a cycle of sexual slavery, there is little hope. No-one knows what happens to those who become so riddled with sexual diseases they can no longer work.

Says Juliet: "It's very hard to find out about it. Nobody knows."

European Union expansion has made it easy to bring the women to the UK, as has the increasing number of cheap flights into regional airports.

Both the Government and police recognise the scale of the problem but concede it is difficult to tackle a trade which is, by definition, an international crime. Cases might have jurisdiction in one country, evidence in another and witnesses in another. Like any organised crime, it is also difficult to persuade victims to testify.

Conviction for trafficking is rare, which is why it has become so alluring to the criminal gangs which can earn as much money trafficking people as they can trafficking drugs. But steps have been made. The Government's new law against trafficking for sexual exploitation came into effect last May, and in December last year, two Albanian traffickers, Taulant Merdanaj and Elidon Bregu, become the first to be convicted of human trafficking in the UK under the law. In the landmark case, Merdanaj was convicted of trafficking people into and within the UK for sexual exploitation and rape, and sentenced to 18 years in prison. Bregu was sentenced to nine years for trafficking within the UK. Their Lithuanian prostitutes were forced to work from a flat in Sheffield, proving that the influx of girls is not just supplying the vice dens of London. But their conviction will do little to help the Chinese girls, who disappeared without a trace from their hostel in Newcastle.

"They may have been destined for the Chinese market or for British punters who like Chinese girls," says Juliet. "Or they may have been moved on altogether. Either way, I feel very concerned for their safety and for their lives."

* The National Missing Persons Trafficking Helpline can be contacted on 0808 800 7070.