The organisation, which maintains a rigidly neutral stance in public, took the unusual step of voicing its concerns in uncompromising language after the former detainees, known as the Tipton Three, revealed that they had been beaten, shackled, photographed naked and in one incident questioned at gunpoint while in US custody.

Their vivid account of the harrowing conditions at the camp, as told to their lawyers and published for the first time in yesterday's Guardian, has reignited the debate about the treatment of prisoners and the British government's role in their questioning and detention.

Last night the Red Cross was joined by the Medical Foundation for the Care of Victims of Torture, which argued that if the allegations were true they indicated systematic abuse, amounting to torture.

Sir Menzies Campbell, the Liberal Democrats' deputy leader, called for the Foreign Office to mount a "searching investigation" into what British officials had seen or been told when they visited Guantánamo Bay.

The Tipton Three were captured in Afghanistan and held at the US military base in Cuba for two years, before being released in March without charge.

One man, Rhuhel Ahmed, alleged that an SAS soldier had interrogated him for three hours in Afghanistan while an American colleague held a gun to his head and threatened to shoot him. The trio also said that they had repeatedly complained of abuse to British consular officials.

"Some of the abuses alleged by the detainees would indeed constitute inhuman treatment," said Florian Westphal, spokesman for the International Committee of the Red Cross in Geneva.

"But we can't comment on this publicly since this type of allegation is raised directly in discussion with the detaining authority.

"Inhuman treatment constitutes a grave breach of the third Geneva convention and these are often also described as war crimes."

The organisation is allowed to visit the detainees to ensure they are treated in accordance with the Geneva conventions as long as it does not disclose information about conditions there. It can breach confidentiality in limited circumstances, most importantly, if going public would be in the best interests of the prisoners.

Sherman Carroll, spokesman for the Medical Foundation, said the report rang true in light of revelations about techniques of interrogation and torture elsewhere.

He added: "If [the detainees] had used the word torture, I would agree with that. This is more than 'torture-lite' [stress and duress techniques] ... Guantánamo Bay should be closed down."

But Major Michael Shavers, the Pentagon spokesman on Guantánamo Bay, said the US operated "a safe, humane and professional detention operation".

He added: "All detainees are treated humanely, appropriately and in accordance with the principles of the third Geneva convention.

"We have investigated all the allegations of abuse at Guantánamo Bay and have dealt with them. They have been resolved."

A spokesman for the Foreign Office said the government had always prioritised the welfare of British detainees and had given them the opportunity to express concerns about their treatment.

He added: "During these welfare visits, neither Mr Ahmed, Mr Iqbal nor Mr Rasul has ever alleged to us that they were systematically abused.