Medical personnel committed a "gross breach of medical ethics" by taking part in torture in Guantánamo, a leaked International Committee of the Red Cross document has revealed.

The 40-page confidential report, written in 2007, describes how medical staff working for the CIA monitored prisoners' vital signs to make sure they did not drown while being subjected to waterboarding, during which water is poured over a cloth placed over a person's nose and mouth.

Medical personnel were also said to be present when prisoners were shackled in a "stress standing position". The detainees were "monitored by health personnel who in some instances recommended stopping the method of ill-treatment, or recommended its continuation, but with adjustments", according to the report.

The Red Cross concluded: "The alleged participation of health personnel in the interrogation process and, either directly or indirectly, in the infliction of ill-treatment constituted a gross breach of medical ethics and, in some cases, amounted to participation in torture and/or cruel inhuman or degrading treatment."

As well as the monitoring of specific methods of ill-treatment, the report said, other health personnel were alleged to have directly participated in the interrogation process. One detainee alleged that a health person threatened that medical care would be conditional upon cooperation with interrogation.

Khalid Sheikh Mohammed, the self-confessed mastermind of the September 11 attacks, alleged to the ICRC that on several occasions the waterboarding was stopped "on the intervention of a health person who was present in the room each time this procedure was used".

Mohammed, who pleaded guilty last year to the September 11 attacks, said he gave a lot of false information during the harshest period of his interrogation.

"Im sure that the false information I was forced to invent in order to make the ill-treatment stop wasted a lot of their time and led to several false red-alerts being placed in the US," he told the Red Cross.

In another case, Encep Nuraman (aka Hambali) alleged that a medical person intervened to prevent further use of the prolonged stress standing position, but told him that "I look after your body only because we need you for information".

Walid bin Attash, who had previously had a leg amputated, told the ICRC that when he was forced for days to stand with his arms shackled above his head and his feet touching the floor, a person he assumed to be a doctor would measure the swelling in his intact leg and eventually ordered that he be allowed to sit.

Florian Westphal, head of media at the Red Cross in Geneva, confirmed the authenticity of the document obtained by Mark Danner of New York Review of Books and posted on its website, but declined to comment on the contents of the report. "It is a legitimate document. It is extremely unusual for an ICRC document on detention procedures to be leaked publicly," he said.

"We regret this as it is important for us to be able to discuss matters confidentially with governments, which gives us the credibility to influence them."

Besides descriptions of how the men were tortured, the report conveys the impatience and frustration of the Red Cross in trying to extract information from the Bush administration. The Red Cross made its first written interventions to the US authorities in 2002, requesting information on the whereabouts of people allegedly held by the Americans in the context of the fight against terrorism.

"Despite repeated requests at various levels of the US government, the ICRC has not received a response to most of these written interventions," the report said.

It took four years once the Red Cross first raised the issue with the Bush administration before it was given access to 14 detainees at Guantánamo, including Mohammed. The report welcomed the decision to grant access to the men, but "deplores the fact that these persons were held in undisclosed detention during a prolonged period by the US authorities and the conditions of treatment to which they were subjected during the time".

The methods of ill-treatment alleged to have been used, the report said, included waterboarding, standing naked with arms extended and chained above the head for periods of two three days continuously, beatings by the use of a collar held around the detainees' neck to bang heads and bodies against the wall, prolonged nudity for weeks or months and prolonged shackling.

Those who were shackled "had to urinate and and defecate on themselves and remain standing in their own bodily fluids for periods of several days".

While the report described practices that have been repudiated by the Obama administration, an Red Cross official who wished to remain anonymous said it was "important for today's authorities to have this information from an independent source".