The Government has been accused of condoning the possible use of torture by the United States as the row over "rendition" flights deepened.
Tony Blair was under pressure to make a fresh statement over the use of British airspace and airports after opposition parties claimed a leaked Whitehall memo showed his Commons statements on the issue may have given a misleading impression.
The Prime Minister's comments on the matter - the "rendition" of suspects to countries that may allow them to be tortured - have hinged on the denial by Condoleezza Rice, the US Secretary of State, that America has breached its obligations under international law.
But opposition MPs said yesterday that Britain had greater obligations than the United States to prevent torture because it had signed the European Convention on Human Rights as well as the United Nations Convention Against Torture.
The MPs warned that Ms Rice's definition of "torture" might allow cruel, degrading or inhumane treatment such as sleep deprivation, exposure to loud noise or indefinite detention without legal process, which are outlawed in Britain but not the US.
The memo from the Foreign Office to Downing Street, which was leaked to New Statesman magazine, advises No 10 to rely on Ms Rice's statements.
Mr Blair duly told the Commons last month: "It [rendition] must be applied in accordance with international conventions and I accept entirely Secretary of State Rice's assurance that it has been."
But the memo points out that the UN convention bans torture but not "cruel, degrading or inhumane" (CID) treatment.
In a section which asks whether "extraordinary rendition" is legal, the Foreign Office says: "In the most common use of this term (ie, involving real risk of torture) it could never be legal because it is clearly prohibited under the UN Convention Against Torture (CAT). But the CAT prohibition on transfer applies to torture only, not to CID. (This may explain the emphasis on torture in Rice's statement)."
The memo also highlighted a loophole in US law, which could allow it to apply a less rigorous definition of CID by relying on the US constitution rather than international treaties.
Nick Clegg, foreign affairs spokesman for the Liberal Democrats, plans to table Commons questions to shed light on the differences between the human rights obligations of Britain and the US.
Downing Street argued the memo had been overtaken by events since it was written. It was confident that no new cases of rendition in UK airspace would come to light. But the Foreign Secretary, Jack Straw, will make a further written statement today.
Accusing the media of "making a meal" of the memo, Mr Blair's spokesman said: "Anything we do in relation to rendition is in compliance with our international obligations. We fulfil our legal obligations."