Those detainees were under 18 when they were captured by US forces, and at least 10 of them still being held at Guantanamo were 14 or 15 when they were seized, held in solitary confinement, subject to repeated interrogation and allegedly tortured, the charity Reprieve was reported as saying.

Britain's Independent on Sunday, which carried the allegations, suggested the charges could threaten the United States' relationship with its closest ally in the "war on terror", Britain.

"We would take a very, very dim view if it transpires that there were actually minors there," it quoted a British government official as saying.

Unnamed government sources said the allegations directly contradicted Washington's assurances to London that no minors were held at Guantanamo.

Human rights conventions

Reprieve's legal director and a lawyer for a number of detainees, Clive Stafford-Smith, told the newspaper the United States could have broken not only its own laws but all human rights conventions by putting children in adult jails.

Mr Stafford-Smith said that even if it were proved the 10 still held there - who are now all thought to have turned 18 - were involved in fighting, they should be treated differently from adults.

According to the newspaper, Washington has admitted that only three Guantanamo inmates - later freed - were ever treated as children.

The US military also admitted that 17 of those on the first definitive list of detainees released earlier this month were under 18 when they were captured, it added.

But Mr Stafford-Smith said Reprieve had credible evidence from other detainees, lawyers and the International Red Cross that an additional 37 juveniles were under 18 when they were taken to Cuba.

A US Pentagon spokesman quoted by the newspaper said no one now being held at Guantanamo was a juvenile and rejected arguments that normal criminal law applied to the facility.