Though Foreign Secretary David Miliband has said the UK government "opposes toture", Ministers have now admitted that they handed over terror suspects in Iraq to US authorities.
The admission amounts to collusion in extraordinary rendition, which has previously been denied.
"This was rendition," said Shami Chakrabarti, director of the high-profile civil rights group Liberty. "It was transfer of prisoners of a kind which had previously been denied."
Defence secretary John Hutton said that two men detained in 2004 were transferred to US custody and were then transported to Afghanistan, where they remain.
He said he was reassured they had been treated humanely but apologised for past incorrect answers given to MPs.
government admitted last year that two US flights carrying terror suspects for interrogation landed on UK territory in 2002, contradicting previous ministerial statements on the matter.
Mr Miliband said the planes had refuelled on the UK dependent territory of Diego Garcia. He said he was "very sorry" previous denials about rendition flights by among others, former prime minister Tony Blair, had been wrong.