Thousands of vulnerable children are being sexually exploited by organised paedophiles, a charity warned today.
Barnardo's warned of a "hidden" problem in which vulnerable youngsters, many of whom have run away from home, are preyed upon by men while on the streets.
The paedophiles befriend the children, offering them gifts and food, and then persuade the youngsters to have sex with them.
The children are often trafficked between towns and cities where other paedophiles will sexually abuse them, a Barnardo's report, Whose Child Now?, revealed today.
The organisation said more than 1,000 children in London alone had been sexually exploited, but said it did not know the full extent of the problem nationwide.
Martin Narey, chief executive of Barnardo's, said: "We alone have worked with over 1,000 children who've been sexually exploited in just 20 of the 209 local authorities.
"We don't know the true extent of this problem. But we know, however hidden from the public eye it might be, it affects many thousands of children.
"We shouldn't have to do this work. But men are not going to stop the predatory sexual abuse of girls and sometimes boys. We shall not stop trying to thwart such men and help their victims escape from their clutches."
Mr Narey said he was shocked after spending time with the workers helping the children.
He told the BBC: "I was horrified by the vulnerability of children who should be in a caring supportive environment who were alone and isolated.
"And when they are alone and isolated, they are picked up by these men who appear to be nice and kind to them at first, ultimately to make them have sex for paltry sums of money."
The paedophiles typically target children who have run away from home and are isolated on the streets.
Mr Narey explained: "One of the trigger points seems to be children who are going missing.
"There are 100,000 children a year who go missing in the UK. Most of them go home after a few days, but my workers in this area say the children who repeatedly go missing perhaps may sleep rough on the streets, perhaps (their parents) may be complacent about children going missing for the third or fourth time, that's the point at which they are vulnerable.
"An apparently kind man will buy them food, will buy them drink and give then a mobile phone, and then a few months later they will be isolated and alone, sometimes in a different town and sleeping with that man and his friends."
He added that one in six of the sexually exploited children was moved from town to town to increase their isolation.
No national records are kept on the number of children exploited in this way.
Barnardo's is calling for police and authorities across the UK to develop a greater awareness of the problem.
Mr Narey added: "We would be delighted to work with more local authorities and put some of our money into this and to provide the shelter for these children and get them off the streets. We know we can do that, we can succeed."