A second source who is familiar with the Senate report told The Telegraph that it contained several unflinching accounts of some CIA interrogations which – the source predicted – would “deeply shock” the general public.

Dianne Feinstein, the Democrat chair of the Senate Intelligence Committee that authored the report has promised that it will expose “brutality that stands in stark contrast to our values as a nation”. The Senate report is understood to accuse the CIA of lying and of grossly exaggerating the usefulness of torture.

It is being angrily opposed by many senior Republicans, former CIA operatives and Bush-era officials, including the former US vice president Dick Cheney, who argue that is it poorly researched and politically motivated.

The CIA has previously admitted that it used black sites to subject at least three high-value al-Qaeda detainees to “enhanced interrogation” – namely Mohammed, the alleged USS Cole bomber Abd al Rahim al Nashiri and alleged senior Bin Laden aide Abu Zubaydah.

An internal report in 2004 by the CIA’s own Office of Inspector General admitted that Mohammed had been “waterboarded” 183 times and Abu Zubaydah 83 times – but actual details of how the interrogations were administered have never been provided.

When the 109-page CIA report was made public in 2009 following a freedom of information lawsuit, large portions of it remained redacted – or blacked out – including all 23 pages that followed the factual admission that interrogators “applied the waterboard technique” to Mohammed.

An official CIA description of waterboarding in the 2004 report says that a cloth is used to cover a subject’s nose and mouth and is saturated with water for “no more than 20 seconds” before being removed. A stream of water is then “directed at the upper lip” in order to prolong “the sense of suffocation”.

However the report also admits that waterboarding was being carried in a “manner different” from that prescribed in the US military’s standard SERE training manual, but details were not revealed, beyond the frequency of the treatment, which was admitted to have broken guidelines.

Among the additional difficulties for investigators seeking the truth about what happened is the fact that in November 2005 the CIA destroyed some 92 video tapes of its waterboarding and interrogation of Mohammed and the others.

The officer responsible, Jose Rodriquez, was reprimanded but justified his actions by arguing that he feared the tapes would eventually leak to the media, provoking a backlash that would endanger officers’ lives.

The White House and the State Department fear that the Senate report could still cause a backlash and have made preparations for increased security at sensitive sites when it is eventually published.

Despite the destruction of video evidence, however, a third source familiar with the still-classified accounts of the most severe of the CIA interrogations, said that the practices were much more brutal than is widely understood.

“They got medieval on his ass, and far more so than people realise,” the source told The Telegraph referring to the treatment of Mohammed and Nashiri, but declined to provide further details because of the still-classified nature of the material.

Amrit Singh, a lawyer with the New York-based Open Society Justice Initiative and the author of Administration of Torture, a book detailing the Bush administration’s torture policy, said the new details of the CIA excesses should not come as a surprise.

“Given the lengths that Bush-era CIA officials went to cover up the truth, including destroying videotapes depicting waterboarding of prisoners, it comes as no surprise that the torture was more brutal than previously revealed.

“It is, however, something that the American public has a right to know about, and an obligation to reckon with, and these revelations only underscore the urgent need for release of the Senate intelligence committee report,” she said.