Lady Eliza Manningham-Buller could have had no doubt that what she said at a lecture in the House of Lords would receive widespread coverage. She must also have known that it would raise hackles across the Atlantic in what is now a fractious time in the "special relationship" between the two countries on intelligence matters.

We do not know why Dame Eliza chose to speak up now, almost three years since her retirement at the head of MI5. Her successor, Jonathan Evans, is unlikely to have welcomed the intervention while the Americans are still smarting over a cases where they feel they have been let down by the British political and legal establishment.

People who had worked with Dame Eliza stress that she adhered to a strict moral code and may well have felt that it was imperative to expose instances where they had been flouted, especially as it had been done by this country's closest ally. The scenario she presented of President Bush, Vice-President Cheney and Defence Secretary Rumsfeld formulating their policy from watching 24 showed the derision she felt about the moral and strategic judgement of the neo-cons who were then in charge of the world's most powerful country and running the "War on Terror".

Dame Eliza was also keen to establish that her service had never indulged in such mistreatment. "Nothing – not even the saving of lives – justifies torturing people", she stressed. There were, however, plenty of accounts of American abuse of treatment during her tenure – October 2002 to April 2007. According to her the UK did learn of one such case and lodged protests with the US. But what happened to the others – did her service attempt to find out what was going on? Or was the feeling the less known the better.

She also refused to be drawn on the statement of Lord Neuberger in the Binyam Mohamed case that her service had a "dubious record when it comes to human rights and coercive techniques". There is a valid argument that it is difficult to see, from the evidence presented in the case, how the Master of the Rolls could come to that conclusion. Dame Eliza, however, chose not to make that point.

The revelations give a valuable glimpse of tension and division between the US and UK in the intelligence war against an implacable enemy. In doing so, however, it also raises questions about who knew what when abuse was being carried out in the name of defeating terrorism.