Lieutenant-Colonel Yvonne Bradley, an American military lawyer, will step through the grand entrance of the Foreign and Commonwealth Office in London tomorrow and demand the release of her client - a British resident who claims he was repeatedly tortured at the behest of US intelligence officials - from Guantánamo Bay. Bradley will also request the disclosure of 42 secret documents that allegedly chronicle not only how Binyam Mohamed was tortured, but may also corroborate claims that Britain was complicit in his treatment.

But first, Bradley, a US military attorney for 20 years, will reveal that Mohamed, 31, is dying in his Guantánamo cell and that conditions inside the Cuban prison camp have deteriorated badly since Barack Obama took office. Fifty of its 260 detainees are on hunger strike and, say witnesses, are being strapped to chairs and force-fed, with those who resist being beaten. At least 20 are described as being so unhealthy they are on a "critical list", according to Bradley.

Mohamed, who is suffering dramatic weight loss after a month-long hunger strike, has told Bradley, 45, that he is "very scared" of being attacked by guards, after witnessing a savage beating for a detainee who refused to be strapped down and have a feeding tube forced into his mouth. It is the first account Bradley has personally received of a detainee being physically assaulted in Guantánamo.

Bradley recently met Mohamed in Camp Delta's sparse visiting room and was shaken by his account of the state of affairs inside the notorious prison.

She said: "At least 50 people are on hunger strike, with 20 on the critical list, according to Binyam. The JTF [the Joint Task Force running Guantánamo] are not commenting because they do not want the public to know what is going on.

"Binyam has witnessed people being forcibly extracted from their cell. Swat teams in police gear come in and take the person out; if they resist, they are force-fed and then beaten. Binyam has seen this and has not witnessed this before. Guantánamo Bay is in the grip of a mass hunger strike and the numbers are growing; things are worsening.

"It is so bad that there are not enough chairs to strap them down and force-feed them for a two- or three-hour period to digest food through a feeding tube. Because there are not enough chairs the guards are having to force-feed them in shifts. After Binyam saw a nearby inmate being beaten it scared him and he decided he was not going to resist. He thought, 'I don't want to be beat, injured or killed.' Given his health situation, one good blow could be fatal," said Bradley.

"Binyam is continuing to lose weight and he is going to get worse. He has been told he is about to be released, but psychologically and physically he is declining."

It is conceivable that Mohamed himself may shortly return to London, heralding yet another political embarrassment for Foreign Secretary David Miliband, who already faces a tumultuous week over claims that he was keen to suppress evidence of torture.

On Tuesday, the unprecedented dispute between Miliband and the judiciary is set to reignite when High Court judges Lord Justice Thomas and Mr Justice Lloyd Jones decide whether to reopen the case which Mohamed believes substantiates his torture claims.

Meanwhile, in San Francisco, a little-publicised court case into the treatment of Mohamed will open. American civil liberties lawyers are hoping to shine a light on the defence firm that allegedly carried out the practice of "rendition" on behalf of the CIA. Jeppesen Dataplan, a Boeing subsidiary, helped to arrange rendition flights for several terror suspects, including Mohamed, to nations where they claim they were tortured.

The case was originally dismissed after the Bush administration asserted "state secrets privilege", indicating that it would endanger national security - the same argument used by Miliband. However, Obama has repeatedly stressed his willingness to be less secretive than his predecessor and a similar decision would lead to claims that the current administration is bent on suppressing evidence of torture.

Closer to home, the Observer has found evidence suggesting a broader unwillingness by Britain to confront the US over its war on terror programme. The Attorney General says it is "actively considering" possible criminal wrongdoings against MI5 and the CIA, but sources claim the government's senior lawyer has failed, after almost four months of looking into the issue, to request material from the US that may substantiate allegations of MI5 complicity in Mohamed's torture.

Suspicion is also growing that some sections of the US intelligence community would prefer Binyam did die inside Guantánamo. Silenced forever, only the sparse language of his diary would be left to recount his torture claims and interviewees with an MI5 officer, known only as Witness B. Such a scenario would also deny Mohamed the chance to personally sue the US, and possibly British authorities, over his treatment.

But if Mohamed survives to come back to London, his experiences of the past six years promise a harrowing journey through the dark underbelly of the war on terror. For Miliband, the questions concerning Britain's role may have only just begun.