Britain believes the CIA's reported secret transfer of terrorism suspects to foreign countries for interrogation is illegal, according to a leaked government document published today.
The foreign office memo says the practice, known as "extraordinary rendition", "could never be legal" if the detainee is at risk of torture, according to extracts printed in the Guardiannewspaper.
It adds that British cooperation "would also be illegal if we knew of the circumstances", according to the newspaper.
Human rights groups have accused the CIA of running secret prisons in Europe and elsewhere, abducting suspects and transferring them between countries by plane.
President Bush said last month the United States does not secretly move terrorism suspects to foreign countries that torture to get information.
Washington has come under growing pressure to explain why hundreds of flights by CIA planes have criss-crossed the world, stopping in many European countries.
Britain, a key US ally, has repeatedly sought to play down its role in the rendition controversy. Foreign Secretary Jack Straw told parliament on January 10th that Britain has approved only two CIA rendition flights.
However, the leaked document, dated December 7th, 2005, says the CIA may have used British airports more often.
The memo was sent by an official in Mr Straw's department to an aide in Prime Minister Tony Blair's office, the Guardiansaid. It was leaked to the New Statesmanmagazine, and parts were reprinted in several British newspapers today.
The briefing document's author, named as Irfan Siddiq, appears to suggest the British government should seek to sidestep difficult questions over its role in the renditions.
"We should try to avoid getting drawn on detail and to try to move the debate on," he wrote, according to the newspaper.