British military chiefs handed a prisoner to the Americans in "questionable circumstances" following the 2003 allied invasion of Iraq and "appear to have sold the pass" which might have allowed them to protect him, one of the UK's most senior judges said on Thursday.
United States' authorities had "in suitably diplomatic language" declined to return Pakistani Yunus Rahmatullah - who has been held without trial for eight years - so that he could be released, said The Master of the Rolls, Lord Neuberger.
The judge was speaking after British human rights lawyers' attempts to free 29-year-old Rahmatullah ended in disappointment.
London-based legal charity Reprieve, and a law firm, won a legal argument last year when they used a piece of ancient English common law to persuade the Court of Appeal in London to order the release of Rahmatullah.
Reprieve and law firm Leigh Day & Co asked appeal judges to order Rahmatullah's release by granting "habeas corpus" relief - a procedure which requires a court to examine the legality of a detention.
Lawyers, who took instructions from one of Rahmatullah's relatives, said he was being held without trial by US forces at Bagram air base in Afghanistan but remained under UK control as part of a "memorandum of understanding" with the Americans.
They said Rahmatullah had been captured by British troops then handed to the Americans - and successfully argued that the UK Government had the power to ask US authorities to free him.
But earlier this week, appeal judges cancelled the release order they made in December after the Foreign Office said American authorities were not going to "play ball" and British ministers had "reached the end of the road".
And Lord Neuberger, who heard the appeal with Lord Justice Maurice Kay and Lord Justice Sullivan, today explained why judges had decided to discharge the habeas corpus writ they had issued.
"The melancholy truth is that the events since we handed down judgment appear to establish that when the UK defence forces handed over (Rahmatullah) to the US authorities in questionable circumstances in 2004 they most unfortunately appear to have sold the pass with regard to their ability to protect him in the future," said Lord Neuberger, the second most senior judge in England and Wales, in a written ruling.
"That does not mean that the issue of the writ of habeas corpus was a pointless exercise in this case: it performed its minimum function of requiring the UK Government to account for its responsibility for (Rahmatullah's) detention and to attempt to get him released.
"This case is an illustration of the court performing perhaps its most vital role, namely to ensure that the executive complies, as far as it can, with its legal duties to individuals, in particularly when they are detained."
The US defence department told British ministers that the American government felt that Rahmatullah was being "properly detained ... consistent with the international law of armed conflict", said Lord Neuberger.
And the Foreign Office told judges that America "in suitably diplomatic language" had "effectively declined" the request for Rahmatullah's transfer into British custody, he added.
Rahmatullah's cousin Munir Ahmed said, in a statement issued by Reprieve: "After eight years of misery, the British court finally gave us some hope when they tried to free Yunus. So we are bitterly disappointed that he is still in Bagram.
"We still believe the British Government can get Yunus back home to his family, and we desperately hope they keep trying to help us. I would just like the British to say to the US: Yunus is cleared for release, so why can't he go home?"