“Rule out nothing whatsoever that you believe may be effective,” reads one cable to Mr Zubaydah’s interrogators. “Rather, come on back and we will get you the approvals.” Videotapes of Mr Zubaydah’s torture were destroyed in 2005, at the behest of Jose Rodriguez, head of the CIA’s National Clandestine Service.

Last year, the UK Parliament’s Intelligence and Security Committee (ISC) established that MI6 had “direct awareness of extreme mistreatment and possibly torture” of Mr Zubaydah, yet “continued to send the CIA questions to be used in interrogations without seeking any assurances regarding Mr Zubaydah’s treatment in detention”.

The ISC made clear that restrictions on its investigations imposed by the government meant its report could only ever be seen as “provisional”.

In July, the government announced it would renege on its long-standing commitment to hold an inquiry into UK involvement in rendition and torture, a pledge made by former Prime Minister David Cameron. 

Reprieve is exploring legal action to challenge the decision. At the same time, the government also presented an updated version of Whitehall’s Consolidated Guidance – the so-called ‘torture policy’ – that fails to expressly prohibit ministers from authorising action carrying a real risk of torture.

Metropolitan Police detectives are currently investigating whether MI5 and MI6 officers involved in the interrogation committed serious criminal offences.

Reprieve director Maya Foa said: “With each new revelation of torture carried out by CIA officers, the need for an inquiry into British complicity in these abuses becomes more plain.

“If MI5 and MI6 officers knew what Abu Zubaydah was being subjected to, but kept on supplying questions, how much other torture did they turn a blind eye to? Only a thorough investigation of this dark period can bring it to an end, and signal that Britain is a country that absolutely and unambiguously prohibits torture.”