William Hague has written to the last British resident inside Guantánamo Bay promising he is doing all he can to bring him home. The foreign secretary's letter comes as testimony from the prison reveals a new hunger strike is spreading.

Hague has contacted Shaker Aamer to reassure him that attempts to reunite him with his family in London are continuing, a process made increasingly urgent by fresh evidence of the 44-year-old's ailing health.

Following a phone call from the US military prison to Clive Stafford Smith, director of Reprieve, which campaigns for prisoners' rights, Aamer has revealed there are now 29 Guantánamo hunger strikers, including him, of whom 19 are being force-fed. "The hunger strike is back on. The number is increasing almost every day," said Aamer. Yet the US authorities, stung by adverse global publicity from the hunger strike during the summer, appear to be introducing policies designed to bury the news about the latest protest.

Officials have revealed they will no longer release figures to journalists on the strike; the last day fresh numbers were released – Monday – 15 prisoners were protesting.

Aamer, who has been detained in the camp for 11 years, said: "Nobody has come to talk to me, nobody is asking what I am doing, there have been no visits from the BHU [Behavioural Health Unit]. Normally they come, so they are trying to keep this under wraps and pretend nothing is happening this time."

News of the protest coincides with fears that Aamer's poor health means he does not have the strength to endure a prolonged hunger strike. In response to his fragile condition, The US authorities agreed to allow an independent medical assessment, which revealed that Aamer has ailments including rotting teeth, poor eyesight, tinnitus, arthritis, swelling in his leg, kidney pains, heart problems, ringworm, irritable bowel syndrome and an enlarged prostate, although no tests have been undertaken to ascertain whether it is cancerous.

Aamer said: "I haven't seen a doctor for almost two years and they refuse to give me proper vitamins and supplements to help me with my health."

Aamer has been on the latest hunger strike for less than a month and his weight has fallen from 188lb on 8 November to around 158lb now. "According to their rules, they should start to force-feed me at 154lb. That will mean being strapped into the chair twice a day, then the 110cm tube up my nose, the liquid forced into me, and the tube hauled back out. I've been through it before. It's horrid … If I have to, I will try to endure.

"But they are ignoring me. If it's anything like before, they might make me go down to 130lb. I won't pretend I am not afraid of what might happen then. I might lose my heart, or my kidney. I don't want to go as a vegetable, or even in a coffin."

With the British government making it clear they want Aamer back and President Barack Obama's intention to reduce Guantánamo's inmate population, Reprieve believes the stumbling block to his release is the stance of the British security services, alleging that MI5 and MI6 made defamatory statements that led to his imprisonment without trial and to his alleged torture and unlawful detention. Stafford Smith said: "Shaker was absolutely thrilled with the letter from Hague, it shows how a certain amount of personal commitment by someone in power can help someone who has been downtrodden in such a ghastly way.

It reflects very strongly that the government is working hard for Shaker, but underlines that some elements are not playing it straight. If there were no opposition to his release, he'd come home tomorrow."

Has anyone ever managed to find any kind of explanation as to why it's supposedly so complicated - what these complex political issues are that make the President unable to keep his promise of closing that hell down? When he's saying these things, how come there's no interviewer demanding to know the precise reasons why?