Shaker Aamer has claimed in testimony to Metropolitan police detectives that MI5 and MI6 were complicit in his torture

The last British resident being held in Guantánamo Bay may never be allowed to return to his family in London because of an alleged "secret deal" between US authorities, Saudi Arabia and the British security services.

Shaker Aamer, 46, has been in the Cuban detention centre for more than 11 years without charge or trial, and has been cleared for release since 2007.

This month, two Metropolitan police detectives interviewed Aamer, gathering an estimated 150 pages of testimony and allegations that MI5 and MI6 were complicit in his torture. These included claims that a British officer was present while US soldiers tortured him and that MI6 officers made allegations to the CIA they knew to be false, including that Aamer was a member of al-Qaida. His legal team alleges that the US, Saudi Arabia – where Aamer was born – and the UK security services are trying to ensure that he never goes home.

Were he to return, he would almost certainly become a key witness in Scotland Yard's investigation into allegations of British complicity in torture in the post 9/11 era.

Despite Foreign Office pressure to bring Aamer back to his family in south London, it has been confirmed that he has only been officially cleared to be sent to Saudi Arabia, where officials have threatened him with imprisonment. A letter dated 18 February 2013 from William Hague, the foreign secretary, to Aamer's lawyer, Clive Stafford Smith, states: "It is our understanding that Mr Aamer has only ever been cleared for transfer to Saudi Arabia."

Stafford Smith, director of the legal charity Reprieve, said: "It seems highly probable that the British security services are in bed with the Americans on trying to keep Shaker from coming back to the UK, since Shaker is such an important witness against them for their complicity in torture. We can only hope that Hague will hold them to account. Scotland Yard has got a lengthy statement from Aamer about his abuse and British complicity in that abuse. The only way to prevent that going forward is for Shaker to go to Saudi Arabia."

In an exclusive interview with the Observer, via an unclassified phone call to Guantánamo Bay between Aamer and Stafford Smith, he revealed his desperation to return to London. The father-of-four, who is approaching day 70 of a life-threatening hunger strike to highlight his plight, said: "I hope I do not die in this awful place. I want to hug my children and watch them as they grow. But if it is God's will that I should die here, I want to die with dignity."

An online petition calling on the British government to bring him home has more than 115,000 votes, triggering a parliamentary debate on the issue.

On Saturday, Saeed Siddique, Aamer's father-in-law, said: "Shaker did nothing wrong. He has been cleared for release twice by the US government. So why is he still rotting in the hell of Guantánamo Bay? Why can't the British government get him back?"

A Scotland Yard spokesman said that a joint panel involving the Crown Prosecution Service and police had convened to assess allegations of complicity in torture involving British officials. A statement said: "Having assessed 12 cases, it has referred three to the Metropolitan police. The Met has decided to undertake further investigation into these three cases."

The Foreign Office said it remained committed to securing Aamer's return to the UK and that the decision lay with the US government: "The position of the British government remains that Aamer should be returned to the UK."