Girls are often threatened with violence by men who had befriended them
Children as young as 10 are being moved around the UK to be sexually exploited at parties organised by paedophiles, a charity says.
Barnardo's says thousands of girls and boys are at risk of organised trafficking.
The organisation says the vast majority of local authorities do not provide expert help for such children.
But directors of children's services insist councils are well aware of their responsibilities - and taking action.
In its report, Whose Child Now?, Barnardo's says although there are more than 200 local authorities across the UK, only 40 are known to provide specialist services for the victims of sexual exploitation.
Barnardo's runs just over half of those services. In those areas, it has been working with more than 1,000 children who were sexually exploited in the past year.
Lisa Stacey, who wrote the report, told BBC News: "Wherever we have been asked by a council to carry out research we have come across young people who have been exploited.
Martin Narey, Barnardo's chief exec: "I was horrified by the vulnerability"
"This is organised sexual abuse and it can often involve the victim being moved from one place to another. More than half the victims regularly go missing."
She says the victims may return after several days to their family or to their care home and say things like "I've been staying with friends". They are often threatened while away and so do not ask for help when they return, she says.
The children are "basically brainwashed", says Ms Stacey.
"Some will even believe that the man who is grooming them is their boyfriend. It can take some of them a year of counselling to realise that they've been abused.
"We're talking about gangs of men, perhaps with a legitimate front.
"In Scotland, we discovered children being brought from outlying areas into cities. We know of children being moved from north-east England to London, or from Yorkshire to London or Manchester."
I felt looked after, wanted, loved even. He gave me everything I wanted
The charity has released the stories of two victims.
Imogen was taken into care shortly after her 12th birthday. She told the charity that she tagged along with older girls who regularly ran away to boyfriends' flats and houses.
She said: "One man started to take a special interest in me. He was much older. He was protective.
"I felt looked after, wanted, loved even. He gave me everything I wanted and, when I was 13, he handed over the keys to a flat and said 'It's yours - use it when you need it'."
But one night she was asked to "dress up" because they were going to a party. She was taken miles from her home to London and told to have sex with different men.
She said: "I didn't have any choice. I felt so guilty. He'd take me all over the country - Leeds, Bradford, Manchester, London. He'd take me to hotels, sometimes for two or three nights.
"I wanted to escape, but he just controlled me. It was a mental thing. I was terrified."
Barnardo's said Imogen's abuser was eventually arrested and her life was slowly turned around. She went back to school and then on to university.
Susan was 13 when her mother died. She didn't get on with her father and so she moved out and began drinking.
He got what he wanted and he wanted to use me. I drank more and more to block it out
She was introduced to the father of a friend, who was 43: "He paid for all my drink. I suppose he did it to keep me quiet. Soon I didn't care about anything. He got what he wanted and he wanted to use me.
"I drank more and more to block it out."
One day she was taken to a flat and ordered to sleep with other men. When she refused she was beaten until she gave in.
Barnardo's said her abuser was arrested and Susan was put in touch with the charity. She now has a flat and is planning to go to college.
Colin Green, who leads on child protection for the Association of Directors of Children's Services in England, said: "Exploitation covers a wide range of behaviour from children being worked by pimps to girls being exploited in return for a ride in a fast car.
"We will be picking up vulnerable children because they may be in touch with a range of services.
"They could come to the attention of youth workers, social workers and police. Some may be doing poorly in school, others may have mental health or emotional problems.
"There will be hot spots, often in the big cities, and councils need to judge how best to respond in their particular area."
The Department for Children, Schools and Families said it had recently published new guidance for local authorities and introduced dedicated child abuse investigation units into all police forces.
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