Mr Mohamed, a British resident, claims he was tortured
A US military lawyer is travelling to Britain to urge the Foreign Office to press harder for the release of a UK resident detained at Guantanamo Bay.
Lt Col Yvonne Bradley told the BBC that her client Binyam Mohamed was very ill as a result of a hunger strike.
Charges against him were dropped last year. He has alleged torture, and the UK government has been criticised after the full details were kept secret.
"Mr Mohamed needs to be released now and not later," said Lt Col Bradley.
When asked why he was still at the camp, despite charges being dropped and the UK government saying it was pressing for his release, she said: "That's the million dollar question. He should not be there."
Ethiopian national Mr Mohamed, 30, is the last recognised British resident to be held in the US detention camp in Cuba.
He has maintained that the evidence against him was obtained through torture, which the US denies.
Lt Col Bradley said there was continuing concern over claims that he was still being detained because he was at the centre of a British High Court case in which he is seeking to prove he was tortured.
Speaking to BBC Radio 4's Today programme before she boarded a flight to the UK, she said she believed there was still a need among US officials to "cover it up".
"My coming to the UK is to try to talk to as many officials as possible and make them aware that Mr Mohamed needs to be released now and not later, and not continue to allow the politics of this case to ruin him," she said.
Last week UK foreign secretary David Miliband disputed claims by two High Court judges that the US had threatened to stop sharing intelligence with the UK over Mr Mohamed's torture case.
Mr Miliband has since been urged to clarify details of the case. The Tories said he needed to explain why he had not asked the new US government to allow intelligence material about the man's treatment to be published.
The High Court judges had said they wanted to publish it but claimed the US had threatened to withdraw intelligence co-operation if it did so.
Mr Miliband said he was not prepared to "join a lobbying campaign" against the US government on the issue.
Lt Col Bradley said the case was not a matter of national security interests, but one of "avoiding being embarrassed by obvious violation of human rights rules".
She said it was embarrassing for the US to "admit" what had happened to him, including his transportation from Pakistan to Morocco, where she said he was "horrendously tortured".
Any level of complicity by the UK would only come out through an investigation and full disclosure of Mr Mohamed's story, she said.
"I think anyone who has a hand in this, whether it's US or UK or any other government, of course does not want this type of information to come out."
Mr Mohamed has been on hunger strike since 5 January and is being force-fed.
"He's in a very poor condition. I was totally shocked when I walked into the cell, to see how thin he was," said Lt Col Bradley.
"It's clear on walking in and seeing the condition of my client, there's a crisis occurring."
She said she believed there were at least 50 detainees on hunger strike.
Mr Mohamed had lived in the UK since he was 15, but was arrested in Pakistan in 2002 and later transferred to Guantanamo Bay.
He was accused of conspiring with Al-Qaeda leaders to plan a series of terrorist attacks.
But a row erupted over intelligence material about his treatment, given by the US to the High Court.
The judges said they had been informed by lawyers for the foreign secretary that the threat to withdraw intelligence co-operation remained, even under President Barack Obama's new administration
But in a statement to MPs last week, Foreign Secretary Mr Miliband said the word "threat" was the judges' own, and all the US authorities had done was "a simple affirmation of the facts of intelligence co-operation".
British firm Leigh Day & Co, which represents Mr Mohamed, has lodged an application with the High Court requesting that the judgement be reconsidered.
The UK attorney general has been asked to investigate claims Mr Mohamed was tortured and British agents were complicit in it.