Terror suspects held by the CIA were interrogated on the British‑owned island of Diego Garcia despite the repeated denials of London and Washington that any such incidents took place, a senior American official said today.

Lawrence Wilkerson, who was the chief aide to former US Secretary of State Colin Powell, said the remote UK-administered military base in the Indian Ocean was used as a back-up location for “nefarious activities”, such as the questioning of prisoners in the wake of the 9/11 attacks, for weeks at a time.

It is the first time that a top Bush administration official has gone on the record to state that the British territory formed part of the chain of “black sites” from Poland to Afghanistan used by the CIA for interrogation and torture.

In an interview with the Vice News website, Mr Wilkerson said that Diego Garcia did not host a permanent CIA prison but was used as a back-up location to conduct interrogations and it was “difficult” to believe they could have taken place without the knowledge of the British authorities.

CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings Show all 12 1 /12 CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings September 2001 Following the 9/11 hijackings by Al-Qaida, US President George Bush signs a Memorandum of Notification that authorises the CIA to capture, detain, and interrogate figures associated with terrorist organisations. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings October 2001 The Office of Legal Counsel authorises the use of military force to combat terrorist activities within the United States. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings January 2002 Military guards take first 20 detainees to the Guantanamo Bay detention camp, located in south-eastern Cuba. The prisoners are classed as “enemy combatants” and therefore not subject to the same legal rights as prisoners held under the Geneva Convention. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings 2002 and 2003 Al-Qaida suspects Khalid Sheikh Mohammed, Abu Zubayda and Abd al-Rahim al-Nashiri are all waterboarded. EPA CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings June 2004 The Supreme Court makes a ruling that reverses a decision saying that Guantanamo Bay lies outside the jurisdiction of the US courts. Detainees now have the right to legally challenge their imprisonment. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings May 2005 Amnesty International brands Guantanamo Bay the “gulag of our times” in its international report. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings December 2005 The Detainee Treatment Act of 2005 is passed. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings February 2006 The United Nations calls unsuccessfully for Guantanamo Bay to be closed. It claims some aspects of the detainees’ treatment amount to torture. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings December 2007 The CIA admits that it destroyed videotapes made in 2002 that evidenced treatment of Guantanamo Bay detainees. Getty Images CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings January 2009 Newly-elected US president Barack Obama pledges to close Guantanamo Bay within 12 months. He later renegades on the bid. GETTY IMAGES CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings December 2013 The Report of the Detainee Inquiry is published. Chairman Sir Peter Gibson concludes that British intelligence officers were aware of detainees’ mistreatment. CIA 'torture' report: Timeline from 9/11 to Dianne Feinstein's findings December 2014 The Justice Department asks the US appeals court to overturn a decision to allow the release 32 videos that depict Guantanamo guards forcibly removing a Syrian detainee from his cell and subjecting him to forced feedings. The Senate Select Committee on Intelligence, chaired by Dianne Feinstein, releases its report.

He said: “What I heard was more along the lines of using it as a transit location when perhaps other places were full or other places were deemed too dangerous or insecure, or unavailable at the moment.

“So you might have a case where you simply go in and use a facility at Diego Garcia for a month or two weeks or whatever and you do your nefarious activities there. No one has indicated there was a detention site there, not in so many words. What they indicated is that interrogations took place there.”

The testimony, based on information from four separate intelligence sources that include a CIA official who participated in Washington’s extraordinary rendition programme, flatly contradicts years of denial on both sides of the Atlantic.

The former Foreign Secretary David Miliband was forced to admit in 2008 that there had been two incidents in 2002 when CIA flights carrying detainees had refuelled on Diego Garcia, the largest of the Chagos Islands where the US has operated a base since 1973 after Britain forcibly removed hundreds of islanders. Mr Miliband said the suspects never left the plane.

Sir Menzies Campbell, a member of the parliamentary Intelligence and Security Committee, called for Mr Wilkerson’s claims to be investigated “with full rigour”. He said: “If true, they would constitute a very different picture to that which has previously been painted of the use to which Diego Garcia has been put by the United States.”

Wilkerson confirmed interrogations took place at Diego Garcia (Reuters) (Getty Images)

Cori Crider, a director at the legal charity Reprieve, said: “This suggests the UK Government has not told the whole truth about Diego Garcia’s part in the CIA’s torture programme. Ministers have consistently said that only two CIA rendition victims ever landed on Diego Garcia. Mr Wilkerson’s comments suggest that either they haven’t been honest with the public or the US government hasn’t been honest with them.”

The charity said the revelations should increase pressure on the Foreign Office to publish logs recording every flight in and out of Diego Garcia. It also called for negotiations over an extension to Washington’s lease on the island, due for renewal next year, to be halted until the truth had been established.

Mr Wilkerson, 70, who served as chief of staff to Mr Powell throughout the Iraq war, said he had not learnt of the CIA’s alleged use of Diego Garcia until after he stepped down in 2005.

He said that on the basis of his own experience while serving on the island in the 1980s and information from his sources, he believed it to be unlikely that any interrogations could have happened without the knowledge of British liaison staff who are in command of the base.

If it can be proven that the British authorities knew about renditions and interrogations on Diego Garcia, the Government would come under intense pressure to establish what levels such knowledge reached. The former Prime Minister Tony Blair and Foreign Secretary Jack Straw have previously denied any use of the coral atoll for rendition.

It was reported last year by Al Jazeera that the Senate Intelligence Committee report, which provided an account of torture by the CIA, would confirm Diego Garcia was used for rendition “with the full co-operation of the UK”. When the document was published the locations of black sites had been redacted.