The former CIA inspector general, John Helgerson, has confirmed that the Bush administration authorised the CIA to use a harsh interrogation method on terror suspect Abu Zubaydah before written legal clearance was given.

This is politically explosive, because the Bush administration has always claimed that it used harsh interrogation techniques such as waterboarding only after government lawyers had determined they did not amount to torture.

This summer, Panorama, in the investigation Licence to Torture, uncovered its own information that Abu Zubaydah had been waterboarded before written legal clearance came. Former CIA agent John Kiriakou, who led the team that captured Abu Zubaydah and was monitoring the cable traffic on his interrogation from CIA headquarters, told Panorama the suspect was waterboarded as early as May or June of that year.

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The controversial government legal opinion that determined waterboarding was not torture was written in August of that year.

The date of Abu Zubaydah's waterboarding is a closely guarded secret that no second source would confirm at the time.

John Helgerson, who has spoken now that his classified investigation into the CIA interrogation programme has been released, made his comments in a recent interview with Der Spiegel magazine.

In the interview the reporter stated that Abu Zubaydah had been subjected to "enhanced interrogation techniques", as practices such as waterboarding are known, before August 1, 2002. "Did the lawyer who signed the memorandum simply authorise a technique months after this technique had already been applied ?" the reporter asked Helgerson..

"You are basically right" said Helgerson. "There was some legal advice given orally to the CIA that had then been followed up by memorandums months later".

In the course of his highly classified investigation into the CIA's interrogation programme launched in 2003, Helgerson interviewed more than 100 people and reviewed more than 38,000 documents.

Much of his report is still blacked out, but the parts that have been declassified found evidence of a variety of interrogation abuses including staging mock executions, intimidation using a powerdrill and threatening to kill the detainee's children in the event of further attacks on the US.

President Obama's administration has appointed prosecutor John Durham to begin preliminary investigations into whether any of the CIA interrogations of terror suspects were illegal. Civil rights groups are calling for the investigations to probe officials at the highest level, as the White House was closely involved in authorising the CIA's programme.

President Obama, however, has signalled that he does not want to be seen to be conducting a witchhunt of his predecessors in government, and it is widely expected that lower level officials will be the ones under the spotlight for now.

