“In fairness, the environment was different then because we were closer to September 11,” said a US official present during the briefings. “But there was no objecting, no hand-wringing. The attitude was, ‘We don’t care what you do to those guys as long as you get the information you need to protect Americans’.”

GOP lawmakers and Bush administration officials previously said members of Congress were well-informed and supportive of the CIA’s use of harsh interrogation techniques. But the details of who in Congress knew what, and when about waterboarding have not previously been disclosed.

In September 2006, the CIA for the first time briefed all members of the House and Senate intelligence committees, producing some heated exchanges with CIA officials, including Director Michael Hayden. The CIA director said during a TV interview two months ago that he had informed congressional overseers of “all aspects of the interrogation programme”.

The US military has officially regarded the tactic as torture since the Spanish-American War. In general, the technique involves strapping a prisoner to a board or other flat surface, and then raising his feet above the level of his head. A cloth is then placed over the subject’s mouth and nose, and water is poured over his face to make the prisoner believe he is drowning.

During these sessions, the agency provided information about the techniques it was using as well as the information it collected.

In May 2007, four months after Democrats regained control of Congress, four senators submitted written objections to the CIA’s use of that tactic and other, still unspecified “enhanced” techniques in two classified letters to Hayden last spring.