A British hunger striker inside Guantánamo Bay has laid bare the deteriorating conditions of inmates, expressing fears that he and others will soon die as a result of what he described as "systematic torture".

Shaker Aamer, the last UK resident still held at the camp, claims he is subjected to harsh treatment from guards and denied water, despite being in a weakened state due to severe weight loss, according to a written declaration filed by his lawyer.

He also alleges that the US base will soon be dealing with its first fatalities as a result of the current action: "I might die this time," he is quoted by his lawyer as saying, adding: "I cannot give you numbers and names, but people are dying here."

The testimony, seen by the Guardian, will form part of evidence presented at a hearing Monday into complaints from the remaining inmates at the notorious detention camp. Over the weekend, clashes broke out as guards attempted to break the hunger strike, which is thought to have begun on 6 February.

Clive Stafford Smith, director of legal action charity Reprieve and Aamer's lawyer, spoke to his client Thursday. Their hour-long conversation was written up as a declaration to go before US district judge Thomas Hogan, who is conducting this week's evidentiary hearing.

Undated handout photo of Shaker Aamer with two of his children Photograph: PA

In it, Stafford Smith quotes Aamer, pictured, describing the dramatic weight loss of inmates since the hunger strike began: "One detainee has lost 55lbs, more than 25% of his weight," Shaker told his lawyer.

As to his own plight, Aamer says he is constantly weak, to the point that he is collapsing on a regular basis.

"I can't read. I am dizzy and I fall down all the time. I do not call (the guards), as it is humiliating. When they call Code Yellow (for when a prisoner collapses), they step on your fingers, your hands, they scratch you, even then you are living in fear when they say they are treating you.

"Yesterday they tied me on the board and they threw me in a cell because the medical people were busy. So they only took me to another cell. You are lucky if you get a medical space."

Shaker has now spent more than 11 years at Guantánamo Bay, despite official UK protest. He had been picked up by the Northern Alliance in Afghanistan in late 2001 and handed over to the Americans.

According to leaked files, the US believes the British resident met with Osama bin Laden during his time in Afghanistan and led a unit of fighters against Nato troops. They also allege an association with shoe bomber Richard Reid and convicted 9/11 plotter Zacarias Moussaoui.

But his supporters claim he suffered terrible abuse at Bagram air base and that an alleged confession he made to captors was obtained under the pain of torture.

Aamer has never been charged or faced trial.

Moreover he was cleared for release by the Bush administration in June 2007, a decision that was re-affirmed three years later. The British government has lobbied Washington on his behalf, stating on Sunday: "we want him released and returned to the UK as a matter of urgency". The UK Foreign Office said it continued to monitor Aamer's condition but stressed any decision to release him remained in the hands of the US government.

In the meantime, the father of four continues to languish in a cell. He has never seen his youngest son, Faris, born while Aamer was in captivity, now living with family in London.

In comments to his lawyer, he expressed a fear that injuries resulting from his experiences at Guantánamo Bay may mean that he will now never pick up the child.

"My back and my neck are getting worse day by day. I don't want the end of this torture here to be paralysed. I want to carry my kids when I get home; I don't want my kids to have to wash me," Stafford Smith reported his client as saying.

Aamer alleges that he is subjected to forcible cell extractions (FCEs) from his cell on a daily basis, during which time he suffers injuries as a result of roughness from the prison guards.

"I have bruises on my legs, knee, my arms where they carry me," he told his lawyer.

He also claims to be at risk of dehydration, as guards won't give the hunger striker water other than at dinner time – when plates of food are left in his cell for hours in a bid to break his resolve, it is claimed

"For three days now if I say I want more water – they FCE me, just to give me water. The first day I got FCE'd three times and Code Yellow two times (when Shaker fell down unconscious)," Stafford Smith quotes his client as saying to him.

During their conversation, Aamer conveyed a message to Stafford Smith to pass on to his wife in the event of his death, and the lawyer believes that his fears are genuine.

He told the lawyer that he was "dying inside" and that those around him were close to death.

"In the night people are dying from the cold. In the day they are dying from the heat. People cry from the heat and humidity – I could not put the prayer schedule on the wall because it was so damp. It is systematic torture," Aamer told his lawyer.

In an indication of the deteriorating condition of inmates, guards took action to move them out of communal areas over the weekend and into single, solid-walled cells in a bid to monitor them more closely.

The action sparked clashes, with prisoners fighting the move with the use of makeshift weapons.

During the confrontation, US troops fired four "less-than-lethal rounds". No major injuries were reported.

The clashes comes amid increased tension between guards and inmates at the centre. It is thought the hunger strike broke out as a protest against the indefinite detention of prisoners as well as tighter restrictions and intrusive searches of Qur'ans for contraband.

Of the 166 detainees still at the base in Cuba, 43 have been classified as hunger strikers, but lawyers claim the true figure of those participating is far higher. Of those, around a dozen are being force fed to keep them alive.

Aamer is not amongst those being forced fed, although he says his weight has fallen dramatically in recent weeks.

"No matter how much I show you I am tough, really I am dying inside," he told Stafford Smith.

• This article was amended on 19 April 2013. The original described Shaker Aamer as a father of three.