Care workers have dealt with a girl who was eight years old, and it is not uncommon for counsellors to get referrals of cases involving children of 11.
A Home Office report reveals that around 70 per cent of people involved in prostitution - both adults and children - have been in care.
The children are often targeted by abusers at pupil referral units and care homes as well as in schools, according to Siobhan Bas, of Barnardo's young women's project in north London. "Children in care are particularly vulnerable," she said. "They have been failed early on in life and have had no continuity of care.
"They have not had experience of reliable adults - they don't know who to trust and they want someone to love them. They are very needy and do not always realise what a loving relationship is."
The project she works for provides counselling and support for sexually-exploited girls.
"It is not uncommon for us to get referrals of 11-year-olds," she said. "We have even had a girl as young as eight."
She said many children were not prostitutes in the accepted sense but victims of abuse and exploitation who were generally kept out of view, in flats and bed-sits belonging to "boyfriends".
"Sexual exploitation is a hidden form of abuse," she said. "There is still a lot of misunderstanding about the issue. We all need to get our heads around it so we can do more to support these kids."
Ms Bas believes that schools and teachers have an important role to play in preventing children from suffering such abuse. "I go in and out of schools all the time and generally I'm impressed," she said. "They are often very good at spotting children who are at risk. But more needs to be done earlier on to educate children about what healthy relationships are and to boost their self-esteem. If we expect teachers to be frontline staff and refer children to organisations like ours, then they must also be given the training to do so."
Fiona Kidd, of the Children's Society, which is staging an exhibition of photographs by teenagers who have suffered sexual exploitation, agreed.
"Early intervention is really important," she said. "You have to be able to recognise the signs that a child is being abused. Teachers need training so that they can do this."
www.the-childrens-society.org.uk, 0845 3001128