In a sworn affidavit, Heather Cerveny, a 23-year-old Marine Corps sergeant, says she met several prison guards at a club on the base where they told her over drinks of harsh abuse of detainees, she said the guards claimed the abuse was both commonplace and justified. The allegations are now being investigated by US Southern Command, under whose jurisdiction Guantanamo Bay falls.

In an interview with ABC News, Sgt Cerveny recounted how one soldier specifically told her: "I took the detainee by the head and smashed his head into the cell door." She alleged that the guard also said: "That guy was annoying me. You know, I smacked him in the head." She said she then asked one of the guards who boasted of the beatings what consequences he had suffered for his part in abusing prisoners. "And he [the guard] said 'nothing'." And, Sgt Cerveny added, "this wasn't bragging like boy talk at the bar. I took it like this is something they do."

The affidavit was filed last week with the Pentagon's inspector general, in accordance with the military's policy of encouraging servicemen and women to report such instances of abuse, after the storm of domestic and international criticism of the treatment of prisoners at Guantanamo and the Abu Ghraib jail near Baghdad. The inspector general referred the affidavit to Southern Command.

The allegations are the latest in a long series of such accounts of detainee abuse, and far from the most shocking or graphic. But they are particularly embarrassing because they contradict recent White House and Pentagon assurances that the problem had been dealt with.

Only last month, as he lobbied Congress to pass legislation setting out new rules for the detention and trial of terrorist suspects, President Bush declared that US troops "can take great pride" in the work they did at Guantanamo Bay, whose 450-plus inmates now include the 14 top suspects transferred there in September after years of detention incommunicado in secret CIA prisons overseas.

It was confirmed this week that this group - which includes Khalid Sheik Mohammed, the alleged mastermind of the 9/11 attacks, and Abu Zubaydah, another high ranking al-Qa'ida operative - received a visit from the Red Cross, their first encounter with non-US officials since their capture. Others however have been held at Guantanamo itself for more than four years, ever since the prison opened in early 2002. Only 10 have been charged with any crime.

Despite assurances from Mr Bush himself that he would rather Guantanamo was shut down, there is no prospect of that happening any time soon. Brushing off the latest such demand - from the British Foreign Secretary Margaret Beckett, US officials reiterated yesterday that those detained were serious terrorist threats, and that the prison would stay open as long as neccessary.

"Look, we don't want Guantanamo open forever," said Sean McCormack, the State Deaprtment spokesman. "We don't want to be the world's jailers. We certainly would look forward to the day when Guantanamo is closed."

Mrs Beckett is the highest ranking British official to publicly criticise the US for its detention of suspects without trial at the camp. Guantanamo Bay did as much to radicalise extremists as it did to promote security, she said. "The continuing detention without fair trial of prisoners is unacceptable in terms of human rights, but it is also ineffective in terms of counter-terrorism."