The Government spent £274,000 on preventing the last Briton in Guantanamo gaining access to files that might have proved his innocence.

On the 10th anniversary of prisoners first arriving at the Cuban naval base, the Wandsworth Guardian can also reveal Mr Aamer's lawyers fear he may die without ever meeting his son.

Mr Aamer, from Battersea, has been held for a decade without ever being charged with any offence.

A Freedom of Information request revealed that since 2009, the Government spent the money on paying treasury solicitors and counsel’s fees and costs for managing the civil case.

In 2009, Mr Aamer’s solicitors won a High Court ruling allowing them access to files which could potentially contain “exculpatory” evidence that “may or may not” be also relevant to Mr Aamer’s claims he was tortured.

The Foreign Office (FO) fought the decision through appeal for months - citing security sharing arrangements with the US - before finally agreeing to let Mr Aamer’s security cleared lawyers have access to the files.

Gareth Peirce, Mr Aamer’s UK lawyer, said 10 years in prison without charge for her client represented a “truly terrible milestone”.

She said: “We remain completely sure that had the British government, last past or present, exerted themselves in any energetic or convincing way insisting he be returned he would be back. We put responsibility on successive UK Governments.

“The coalition Government suggested they would be different and vigorous and they would succeed. They were ensuring his family they were expecting to be successful. They went back to US earlier this year and they have had to confess that wasn’t the case. The promises of the last year have proved to be misjudged.”

She added the UK had underestimated US regulations, which mean congress has to sign off on any Guantanamo prisoner release.

“We were given assurances of the official visits, the level of those visits, specifically about Shaker and the likely success of that. Sadly there appears to have been a complete misunderstanding on the part of the British of the impact of US regulations.

“[President] Obama and [Secretary of State] Hillary Clinton were asked about it during visits, then the UK Government went to Washington with proposals for his immediate return, which the family were told were expected to be successful. Instead the highest echelons of the US said sorry we can’t do it.”

The FO did not answer specific claims but a spokesman said Mr Aamer’s release was raised again the Secretary Clinton “earlier this month”. He said: “We remain committed to securing the release of Mr Aamer, but ultimately it is a decision for the US authorities.”

Defending spending money on the High Court action, he added: “The Foreign Secretary has made it very clear how important it is for the UK’s national security that the UK and US are able to share information in confidence.”

Clive Stafford Smith, who represents Mr Aamer in the US and saw his client in November, said: “This is a guy who has spent 10 years in US custody and been abused as horribly as anyone in Guantanamo. It would take an enormous toll on anyone. He is a human being that is gradually falling apart at the seams.

“He could die there . . . not many people would get 10 years in prison for a murder they did commit, let alone 10 years in jail without charge.”

Shaker Aamer factfile Shaker Aamer was captured by Afghans and sold to US forces around December 24, 2001. He claims he was tortured at a US facility in Bagram, Afghanistan, while a British Secret Service agent was present. Transferred to Guantanamo two months later, he claims there was been periodically tortured – including water-boarding and spending several years in solitary confinement.

The US claim he was fighting with Al Qaeda, but have never brought a charge against him.

Mr Aamer, who had indefinite leave to remain in the UK, maintains he was in Afghanistan helping with humanitarian projects, including building a school, before his capture.

Cleared for release by a military commission in 2007, Mr Aamer has four children – the youngest of whom he has never met - with a British wife.

His lawyers say he is an important witness in two ongoing police investigations into claims British intelligence officers were complicit in torture.