Increasingly brutal tactics are being used in an attempt to break the hunger strike by detainees at Guantánamo Bay, according to fresh testimony from the last British resident still held in the camp.

Shaker Aamer claims that the US authorities are systematically making the regime more hardline to try to defuse the strike, which now involves almost two-thirds of the detainees. Techniques include making cells "freezing cold" to accentuate the discomfort of those on hunger strike and the introduction of "metal-tipped" feeding tubes, which Aamer said were forced into inmates' stomachs twice a day and caused detainees to vomit over themselves.

The 46-year-old from London tells of one detainee who was admitted to hospital 10 days ago after a nurse had pushed the tube into his lungs rather than his stomach, causing him later to cough up blood. Aamer also alleges that some nurses at Guantánamo Bay are refusing to wear their name tags in order to prevent detainees registering abuse complaints against staff.

Speaking last week from the camp in Cuba, exactly four months after he joined the hunger strike, Aamer said: "The administration is getting ever more angry and doing everything they can to break our hunger strike. Honestly, I wish I was dead."

The momentum behind efforts to release Aamer – who has spent more than 11 years without trial inside the camp – mounted sharply last week with David Cameron raising the issue directly with the US president, Barack Obama, during the G8 summit in Northern Ireland.

On Wednesday, in a response to a parliamentary question about what had been discussed by the two leaders, Cameron revealed that his next step would be to write to Obama about the "specifics of the case and everything that we can do to expedite it". He added: "Clearly, President Obama wants to make progress on this issue and we should help him in every way that we can with respect to this individual."

The prime minister's comments are the most positive indication to date that Aamer will eventually be freed – he has been cleared for release twice since 2007.

Clive Stafford Smith, the director of the legal charity Reprieve, who passed a transcript of his conversation with Aamer to the Observer, said: "These gruesome new details show just how bad things are in Guantánamo. The whole thing is at breaking point. Clearly the US military is under enormous pressure and doing everything it can to hurt the men and break the hunger strike."

Although the military initially denied that there was a hunger strike inside Guantanámo, it now concedes that, of the 166 detainees, 104 are on hunger strike and 44 are being force-fed.

Aamer also documents his declining health and how the camp's regime deliberately inflates the weight of detainees on hunger strike. Aamer, who has permission to live in the UK indefinitely because his wife is a British national, said: "They said I was 160lb, but I was 154lb a few days ago. Unless there has been a miracle, my weight has not gone up without eating. But they cheat by adding shackles and sometimes even pressing down as they do it to add to your weight.

"If you have a medical standard for when a detainee should be force-fed for his own health, then force-feed him when it can still save his health. Don't wait until his body is so harmed by the lack of food that all you are protecting is the US military – from the harm of a prisoner dying for a principle."

Aamer describes his daily diet at Guantánamo as a cup of tea or two each day with a low-calorie sweetener and occasionally an Ocean Spray powder mix that has 10 calories – enough to give an energy boost.