Young women tricked into coming to England, often by boyfriends, are being sold off in auctions at airport coffee shops as soon as they arrive.
There is evidence of thousands of children working in the sex trade
They are among the thousands of women brought into the UK to be sex slaves, usually with no idea of their fate.
The trade was one of the findings of a BBC News website investigation into slavery in 21st Century England.
As the UK marks 200 years since the Parliamentary Act to abolish the slave trade, slavery goes on in another form.
The slave trade, outlawed by legislation introduced in March 1807, saw people from Africa transported en masse to the Americas with the involvement of people from the UK and other European countries.
Modern day victims of slavery are often young women from eastern Europe, thinking they are coming to England to work as cleaners or au pairs, only to be forced into prostitution.
The Home Office estimated in 2003 that 4,000 women were trafficked into the UK for sexual exploitation. It is thought the figure may have grown since.
Police forces from Cornwall to Northumbria have found themselves having to rescue women and prosecute the traffickers who brought them to England to work as sex slaves.
And as well as foreign citizens coming to the UK, charity workers in Manchester told the BBC they believed British women working in massage parlours had been sold abroad, because they owed the owners money.
Child protection organisations and human rights groups also believe there are thousands of child sex slaves in the UK.
The UK Human Trafficking Centre was opened last year to co-ordinate the law-enforcement approach to the problem.
A spokesman told the BBC women were sometimes sold off in auctions in airport coffee shops and restaurants as they arrived in the country.
And he said there were also many cases of English women, from backgrounds of poverty, being sold from town to town to work as prostitutes.
But it is now believed that as many as 85% of women working in brothels in the UK have come from overseas - in the mid-1990s, an estimated 85% were UK citizens.
Operation Pentameter, a Home Office initiative aimed at rescuing sex workers held against their will carried out between January and July 2006, saw 84 trafficked women rescued, including 12 aged 14 to 17.
Some 230 arrests were made and more than £250,000 in cash was seized - but officers were only able to visit about 10% of the estimated number of sex establishments in the country.
The Poppy Project, a London-based scheme which provides accommodation and support for the women, has had 581 victims referred to it since its launch in 2003.
Its own research in 2004 found evidence of "off street" prostitution in every one of London's 33 boroughs, again with the overwhelming majority of workers in brothels, saunas and massage parlours being non-British nationals.
The Helen Bamber Foundation, set up to help victims of torture and other human rights violations, said women being forced into sex slavery in England were experiencing "horrific brutality", with physical violence and the psychological trauma of being forced into sex.
Many women rescued from the sex trade have said they were sold, or strongly encouraged into heading for the UK, by boyfriends or family members.
Many think they are coming to work in jobs such as cleaning or ice cream selling.
Jiera, a 19-year-old from Lithuania who was helped by the Poppy Project, thought she was coming to London on holiday with friends, only to find they were people traffickers who sold her into prostitution.
She said: "When I was with clients I tried to pretend I was doing something else, but I couldn't. It made me so angry that I was often violent towards the clients.
"The man who owned me beat me and then sold me on. I was too much trouble.
"Even if my friends don't judge me for what happened, they will always know what I did. They will never forget, and neither can I."
Many police forces - and not only in major metropolitan areas - have set up specific teams to deal with the problem.
It is estimated 85% of UK brothel workers are from overseas
Sussex Police appointed a detective to lead investigations into sexual exploitation in November.
They said they had responded to information concerning potentially exploited women working in brothels in the county almost every week since.
The government prioritised human trafficking during the UK's presidency of the EU in 2005.
In the next few weeks it is to lay out a strategy to counter the problem.
A Home Office spokesman said: "Human trafficking is a particularly horrible crime, based on deceit, exploitation and very often brutality.
"It is a crime that has a devastating effect on the lives of individuals, and contributes to the overall harm caused to the country by organised crime.
"It is important for all countries, including the UK, to do whatever is necessary to develop effective enforcement, prevention and victim support systems, both internationally and domestically."