Child trafficking should be made a criminal offence in the UK, closing a legal loophole, says Unicef.
Children are seen by traffickers as just another commodity
The UN agency says thousands of children are being trafficked to Britain each year and human trafficking is the fastest growing business of organised crime.
The children are being drawn from a growing number of countries - mainly from West Africa, but also from other African countries, Eastern Europe, Asia and even Jamaica.
The issue hit the headlines on Tuesday when 21 people were arrested in the case of 'Adam', the Nigerian boy whose torso was found in the river Thames in 2001.
Police believe a ring of people trafficked the boy into Britain before he was ritually killed.
Trafficking is often being masked by the West African cultural practice of sending children to live with extended family or friends to be educated or to find work, said Unicef.
Between 8,000 and 10,000 children are being privately fostered in the UK, it said - many from West Africa.
This was often for the child's benefit, but "many of these children could be being abused or exploited, without anyone even knowing they are in the country," it said.
Another West African child, Victoria Climbie from Ivory Coast, was tortured to death in 2000 by a London-based great-aunt who used her to claim child benefits.
Towns and cities
Children are now being taken not only to London and other UK capitals, but to smaller towns and cities nationwide where the authorites may be less aware of the problem, said Unicef.
"In some cities such as Nottingham and Newcastle, cases have only started coming to light since late 2002, indicating that the traffickers are widening their operations and trying new places," it said.
Escaped from abusive father in Romania aged 12
Asked family friend for help: He took her to Serbia and sold her
She was prostituted there and then in Macedonia, Albania, and Italy
Resold and brought to the UK, again for prostitution
Escaped into social services' care
Her trafficker sentenced to 10 years in prison in 2002
Unicef said the children were controlled and intimidated in a number of ways - with Albanian gangs using rape, violence and threats against relatives, and West African traffickers threatening voodoo and curses, for instance.
"It's a huge concern for us, not least because of the terrible, appalling suffering which the victims go through," said Unicef UK's executive director David Bull.
"You can imagine almost nothing worse than being forced or tricked away from your family to a foreign country where you're abused and beaten and sexually sold almost on a daily basis."
The Sexual Offences Bill, currently in the House of Commons, will make it illegal to traffic people into the UK for commercial sexual exploitation.
WHAT IS CHILD TRAFFICKING?
Trafficking means transporting and exploiting unwilling or unknowing victims
Child victims often used for sex work
May also be used as domestic servants, drug mules, in sweatshops and restaurants, or as beggars or pickpockets
But children trafficked for other reasons will remain unprotected.
Unicef called on the government to criminalise trafficking for all purposes.
It also urged the government to provide funding for specialist care - including safe house accommodation and counselling for the victims, and training for immigration officers and social workers.