Barnabus, a Christian outreach charity which works with people involved in the sex trade in Manchester, has reported a sharp rise in the number of women from eastern Europe working the streets in the last 12 months.

An estimated 4,000 trafficked women are thought to be working in the British sex trade at any one time, having arrived from across the world.

But according to charities, it is not a threat confined to foreign nationals - some believe it is a two way street and that British women also face the threat of being sold abroad.

Peter Green, project director of Barnabus, told the BBC: "I believe it (sex trafficking) is widespread.

"But I also believe we are not aware this is a two way issue, that young women are being taken out of our country and sold abroad because they owe money to the massage parlours."

At a time when the UK is marking the 200th anniversary of the Parliamentary Act which brought about the abolition of the slave trade, women are increasingly being trafficked. Mr Green has been alerted to the threat hanging over one British woman by fellow charity workers.

"It's something I'd never considered, but when I first heard about the plight of this young lassie it became a logic to me," he said.

"If people are being transported one way they can be transported the other way.

"This young lassie was having to service 20 guys a day just to pay off interest, that's before she started to earn anything, and she was under constant threat of being sent out - trafficked - out of the country."

After raising the issue at a meeting of the Stop the Traffik organisation in Manchester and discussing it with other Church of England charities, Mr Green believes it is an issue that has not been previously considered.

"I don't believe people see it, particularly the church," he said. "We just see what we see on the television and in the newspapers (about trafficking) and we are working from that without looking in the opposite direction."

According to the National Christian Alliance on Prostitution (NCAP), there is "an issue" of UK women being trafficked abroad.

Spokesman Mark Wakeling told the BBC that Mr Green's was "not an isolated story".

"I have heard of it as well, from a group working in the West Midlands, about young girls being groomed by men who try to get them to go overseas to 'visit people'.

"Who knows what happens when they get there? It could be viewed as a form of slavery," he added.

But for Christian groups like NCAP and other charities that work with women involved in the sex trade, the issue of British women being groomed for sexual exploitation within the UK is just as pressing.

Mr Wakeling points to the United Nations (UN) definition of trafficking, which encompasses themes such as the abuse of power or a position of vulnerability as well as the recruitment and transfer of people.

"A lot of people think trafficking only happens when there is a change of country," he said.

"Actually, the vast majority of women on the streets are being exploited because of their vulnerability in the same way someone brought from Lithuania to Manchester is being exploited as well.

"You might not always have a physical enslavement (as a prostitute) in the UK but you certainly have a psychological enslavement."