Documents that could prove, for the first time, British secret service agents were present at the torture of a Guantanamo Bay (GB) detainee will be made available to his lawyers after a late Government u-turn in the High Court yesterday.

The decision means, subject to US also agreeing the UK’s position, lawyers should see previously censored material, which could secure the release of Shaker Aamer, the last British GB detainee - who has been held in the notorious prison camp for nearly eight years.

After the decision human rights lawyer Gareth Peirce, of solicitors Birnberg Peirce & Partners which is representing Mr Aamer in the UK, said the case could have serious implications on how the security services share intelligence with other countries and the courts.

Mrs Peirce, who has represented several high profile cases including the Guildford Four, the Birmingham Six, and other GB detainees, said: “The fact they [the Government] have come to court and are arguing a position is important, as is the fact the court may, one day, compel government to give this type of information to lawyers and the public.

She added: “What we are saying is that the UK has been complicit in appalling crimes, abuses and tortures. There’s a fight to the death in the courts to keep this secrecy. [The case] is important for Shaker Aamer and the British public.”

Mr Aamer, a Saudi Arabian national and Battersea resident who has indefinite leave to stay in the UK, accuses secret service agents of being present when he was tortured in 2001 - and said he only made confessions under duress.

The US alleges he has helped train Al Qaeda operatives and was close to Osama Bin Laden.

Last week lawyers won a ruling In the High Court that documents the UK Government had sent to the US authorities to aid a review or Mr Aamer’s case, should be made public.

The Government, which previously released information on the understanding it wouldn't be shown to Mr Aamer's lawyers, went back to the High Court yesterday, seeking to use a “public interest immunity” argument to block their release.

It asserted full disclosure could jeopardise the workings of the security services and intelligence sharing with the US.

But, at the beginning of proceedings, Lord Justice Sullivan, sitting with Justice Lloyd Jones, announced that “overnight” the UK Government had agreed Mr Aamer’s US lawyers - with the relevant security clearance - could have access to the files.

Mr Aamer's barrister, Richard Hermer QC, welcomed the development but told the court the u-turn showed the Government had not “done all it could” to secure Mr Aamer’s release.

He said: “The assertion that they had done everything they possibly could is incorrect. By sending documents with an express proviso they were not shown to the claimant does not match their claim.”

James Eadie QC, acting for the Government, dismissed the statement and said it was facilitating the US process - where Mr Aamer is accused of being involved with training Al Qaeda operatives.

Lord Justice Sullivan adjourned the High Court case until January to give the US Guantanamo Detainee Review Task Force - part of President Barack Obama’s review of GB inmates which is due to decide whether Mr Aamer should be released next month - time to hand over material.

Adding the case would be made redundant if the material was handed over, he said: “It would seem to us that dealing with the matter in this way might concentrate the minds of those involved with both governments,” adding “it seems this is in the best interests of all parties”.

Mr Aamer’s case also resonates with that of Ethiopian-born Binyam Mohamed - a UK resident and former GB detainee, released earlier this year, who alleges he was tortured while held by US agents before being taken to GB.

The High Court is due to rule in the new year whether the UK Government should release sensitive intelligence documents in his case.

Mr Aamer, who has never been charged with an offence or put on trial, is represented in the US courts by Reprieve, the legal charity which has acted to enforce the human rights for GB detainees.

His US lawyer, Clive Stafford Smith, said the result was “a step forward” but was wary of the concession being used as a delaying tactic - as the US would now have a short time to agree to releasing the documents before the trial.

“To date, everything has been a delaying tactic. One might well ask why anyone would think it appropriate to cover up evidence of torture under any circumstances,” he said. “I'm not sanguine about this. The question is how we can make them do it. Last time, with Binyam Mohamed [also represented by Reprieve] they reached a deal with the US and the US prosecutors still gave us only seven of the 42 agreed documents, and it took another round of litigation in the UK to force the issue.”

His lawyers also remain concerned the US could transfer Mr Aamer back to his native country, where pressure groups like Amnesty International have concerns about human right abuses.

Outside the court Mrs Peirce added: “We know they have already tried to make him board a plane to (Saudi Arabia) once and this could happen again.”

She said: “He is the only British detainee left there. All the others have come back. We are extremely worried why he is the only one not brought back home.”

She said Mr Aamer’s case was important in forcing the Government to make public security information.

She said: “The fact they [the Government] have come to court and are arguing a position is important, as is the fact the court may, one day, compel government to give this type of information to lawyers and the public.

“The countering element is the challenge to the secrecy of the British services. In this case there has been torture and yes British agents were present. We want an explanation.

“What we are saying is that the UK has been complicit in appalling crimes, abuses and tortures. There’s a fight to the death in the courts to keep this secrecy. [The case] is important for Shaker Aamer and the British public.”

The Foreign Office (FO) has made two requests to the US, including one recent request from Foreign secretary David Milliband, asking for Mr Aamer’s release.

It welcomed yesterday’s decision. A spokesman said: “We have made clear to the US authorities that we are content for the material we have provided to them to be shared with Shaker Aamer's security cleared legal counsel in the US. We have always maintained that the US Courts are the appropriate jurisdiction for Mr Aamer to pursue his claims for disclosure.

“We informed the US in a letter Wednesday that for the avoidance of doubt we were content for intelligence material from the UK, that has already been provided to the US Review Panel considering Aamer's case, to be made available to Shaker Aamer's security cleared counsel.”

After last week’s High Court ruling a Government spokesman said: “We will continue to argue strongly the point of principle involved in this case - that it is fundamental to the national interest of the United Kingdom that our intelligence and security services are able to operate without fear of having to disclose secret intelligence material.”

Mr Aamer, 42, was captured in Afghanistan in 2001 and alleges he was visited by both MI5 and MI6 in Kandahar in Afghanistan and that he was interviewed in Bagram, Afghanistan by an MI5 agent “John”, aged about 40, who said that there was “nothing he could do as it was all in the US’s hands.”

“John’s” interrogation allegedly took place after Aamer had been subjected to weeks of torture including sleep deprivation over nine days, cold water torture which led to frostbite, “hog tying” and beatings along with threats that he would tortured in Egypt, Jordan, or Israel. He was transferred to GB in February 2002.

Mr Aamer, in a statement through his lawyers submitted to court last week, said: “Once after a few days of sleep deprivation they took me to the interrogation room and the intelligence team starting coming one after another and the room was full, up to 10 or more.

“One of them, a British MI5 agent, was standing and they started talking to me in different languages - English, French, Arabic - and shouting.

“I felt someone grab my head and start beating my head into the back wall so hard that my head was bouncing. They were shouting that they would kill me or I would die.“ Saaed Ahmed, Mr Aamer's father-in-law, said his daughter had developed health problems since her husband’s arrest. He said: "The court case is good news. It has been eight years he has been away now. His children keep asking where he is. He has not seen his youngest daughter and she has never met her father.”

• What do you think? Let us know by email here, phone the newsdesk on 020 8330 9555 or leave a comment below.