Claims British Security Service agents were present at the torture of the last British resident in Guantanamo Bay are being investigated by the Metropolitan Police, the High Court heard on Friday.

Lawyers acting for Shaker Aamer, who is from Battersea and has been held by US authorities for eight years without charge, said the revelation meant British Government “now had a duty” to ensure he was released.

Met Police detectives applied to the court earlier this month for release of confidential Government files in Mr Aamer’s case to aid the investigation, it was revealed in court.

It was also revealed investigators regard Mr Aamer as a key witness in a separate investigation into allegations British agents knew about the torture of former British Guantanamo detainee, Binyam Mohamed.

Earlier this month, the Appeal Court ordered the disclosure of files showing MI5 knew that Mr Mohamed was being mistreated by the CIA.

At court on Friday, Richard Hermer QC said: “Met Police attended the offices of the solicitors. They are now investigating allegations into alleged complicity in his [Mr Aamer’s] mistreatment.”

He added: “It appears that these investigations are not new, their existence was not previously known to the claimant’s legal team in either the UK or the US.”

Gareth Peirce, Mr Aamer’s lawyer in the UK, said the development compelled the Government to act to bring Mr Aamer home.

“Police are interested in potential evidence that Shaker can provide into both investigations,” she said, “they came to see us to ask for our assistance.”

“The man is a victim and the man is a witness in UK investigations. It [the Government] has a duty to have him here,” Mrs Peirce said. “The one question is why isn’t he [Mr Aamer] back here, why is there so little being done, why is it that there’s no energy being put into bringing him back?”

While the Foreign Office said it was working behind the scenes to free Mr Aamer, there was little evidence of that, she said.

“They [the Government] say they are helping, but where is the help” she said. “This is possibly one of the most important criminal investigation there has been in this country, to investigate complicity and torture.

“This man has been tortured and he is a key witness,” she said, “but where is he, in the Caribbean, but not by choice.”

The Met refused to comment on the investigation but a Foreign Office spokesman said it was continuing to press for Mr Aamer’s release “at the highest levels”.

He said: “There are currently two police investigations into allegations of possible criminal wrongdoing by British government officials.

“In addition to the police investigation into allegations made by Binyam Mohamed, one further case has subsequently been referred by the Attorney General to the police. The case was referred to the Attorney General by the Secret Intelligence Service on its own initiative, unprompted by any accusation against the Service or the individual concerned. These investigations are a matter for the police and the Government cannot comment further.”

The spokesman again denied allegations of UK involvement or complicity in Mr Aamer’s mistreatment.

He said: “We firmly reject any suggestion that we torture or mistreat people or ask others to do so on our behalf. Mr Aamer and his legal representatives have made a lot of unsubstantiated allegations.”

Mr Aamer, a Saudi Arabian national who has indefinite leave to stay in the UK, was captured in 2001 in Afghanistan, where the US claims he was helping the Taliban.

He claims he was tortured in Guantanamo and during his detention in Bagram, Afghanistan, where, he alleges, Britain colluded in that torture by sending agents to interview him.

Mr Aamer has a wife and four children, the youngest of whom he had never seen.