The former head of MI5 has alleged that US intelligence agencies concealed their mistreatment of terror suspects.
Baroness Manningham-Buller said that it was only after her retirement in 2007 that she learnt that Khalid Sheikh Mohammed, who masterminded the 9/11 attacks, had been waterboarded.
Her comments, made in a lecture to the House of Lords, come amid a period of intense controversy over the alleged collusion of British agents with their US counterparts over the use of torture.
She said that the US was "very keen to conceal from us what was happening", adding: "The Americans were very keen that people like us did not discover what they were doing."
Last month, it was disclosed that British resident and former Guantanamo Bay detainee Binyam Mohamed had undergone "cruel, inhuman, and degrading" treatment.
Jonathan Evans, current head of MI5, said that British agents did not collude with foreign agents on this treatment. Questions have been raised about when the UK learnt that America had changed its policy on torture.
On the case of Khalid Sheikh Mohammed, Manningham-Buller said that she had wondered how the US had obtained evidence from him: "I said to my staff, 'Why is he talking?' because our experience of Irish prisoners, Irish terrorists, was that they never said anything.
"They said, well, the Americans say he is very proud of his achievements when questioned about it. It wasn't actually until after I retired that I read that, in fact, he had been waterboarded 160 times."
The foreign office declined to comment.
