The decision by the Bush administration to press for the death penalty against six men accused in connection with the September 11 attacks on the US will place further strain on the relationship between America and Britain over the prosecution of the war on terror.
Britain is opposed to capital punishment and has been increasingly critical of the treatment of prisoners held in Guantanamo Bay.
Both Tony Blair and Gordon Brown have called for the closure of the prison camp at the US naval base in Cuba which still holds 275 inmates, many of whom have been unlawfully detained for more than five years.
The former attorney general Lord Goldsmith also expressed concerns about the legally flawed system of the military tribunals, set up to try non-US citizens and which one law lord likened to "kangaroo courts".
Any convictions supported by findings from the military commissions are bound to provoke an international outcry. Human rights lawyers regard the tribunals as an affront to natural justice because the evidence against the suspects has been secured through torture or unlawful detention.
Reprieve, the UK legal campaigning charity, condemned America's reliance on the tribunals and its latest decision to seek the death penalty against the six men.
Clive Stafford Smith, Reprieve's legal director, said: "Military commissions in Guantanamo Bay are not about justice – they are about politics. The proceedings these men would face are deeply flawed. Someone could be put to death based on secret or third or fourth-hand evidence. That is not the American way."
Reprieve fears Binyam Mohamed, a British resident, could be the next to go before the tribunal. Mr Mohamed claims he was tortured while being questioned in Morocco by US interrogators. His lawyers say his mental health has now deteriorated so seriously he has become a suicide risk. Since the Americans agreed to release four British residents from Guantanamo last year, the Foreign Office has been making representations on Mr Mohamed's behalf.