Tony Blair has defended co-operation with the US over the secret transfer of terrorism suspects, amid fears that some could be facing torture.
Mr Blair said movements had to follow international conventions
The US has been criticised for the practice, but says it does not move people to countries which use torture.
Lib Dem leader Charles Kennedy told MPs that suspects were being transferred to Afghanistan and elsewhere "apparently for torture purposes".
But Mr Blair said "international conventions" had to be applied.
Some of the individuals had to be detained for international terrorism reasons, he said, adding that some were dangerous while others had vital information.
"Torture cannot be justified in any set of circumstances at all," Mr Blair said.
The Bush Administration has come under increasing pressure about the secret transfer of terrorist suspects - a practice known as "extraordinary rendition".
US Secretary of State Condoleezza Rice insisted on Monday that America did not practise torture or transport detainees to places where they believe they would be tortured.
At prime minister's questions, Mr Kennedy demanded to know the extent to which Britain had cooperated with the US over extraordinary rendition.
Mr Blair said it was vital to distinguish between taking a suspect from one country to another and torture.
Extraordinary rendition had been US policy for years, he added.