Secret evidence which might have been obtained by torture cannot be used against terror suspects in UK courts, the law lords have ruled.
The ruling means the home secretary will have to review all cases where evidence from other countries might have been obtained in this way.
It is a victory for eight men who were previously detained without charge. The government says it does not use evidence it knows to have been obtained by torture.
The ruling centres on how far the government must go to show improper methods have not been used. The Court of Appeal ruled last year that such evidence was usable if UK authorities had no involvement.
But eight of the 10 foreign terror suspects who were being held without charge, backed by human rights groups, challenged that ruling. They argued evidence obtained in US detention camps should be excluded.
The Special Immigration Appeals Court (SIAC) must now investigate whether evidence was obtained by torture, the law lords have ruled.
Daily Telegraph legal correspondent Joshua Rozenberg told BBC News 24 the ruling was a “very significant blow for the government”.
He said it would not be enough for suspects simply to say the evidence against them had been obtained under torture – it was up to SIAC to investigate their claims. But if the government was not prepared to say where evidence has come from, it must find other evidence to justify their continued detention.
Shami Chakrabarti, director of civil rights pressure group Liberty, said: “This is an incredibly important day, with the law lords sending a signal across the democratic world that there is to be no compromise on torture.
“This is also an important message about what distinguishes us from dictators and terrorists. We will not legitimise evidence obtained by torture by using it in our justice system.”
Home Secretary Charles Clarke has said he would deport anyone considered a threat to national security. But the suspects have the right of appeal to SIAC.