A senior judge has ordered the government to help the British resident currently awaiting trial for war crimes at Guantanamo Bay.
Binyam Mohamed has claimed he was tortured in Morocco and that British officials submitted information about him to the authorities there which was used against him.
But the government has refused to provide information about what human rights groups term his 'CIA-sponsored torture' in Morocco.
"[T]he UK is under no obligation under international law to assist foreign courts and tribunals in assuring that torture evidence is not admitted," the government argued.
But UK charity Reprieve, together with law firm Leigh Day, sued the British government, demanding it turn over evidence which could help prove Mr Mohamed's innocence and the extent, if any, of his torture.
The court has rejected the government's argument.
Mr Justice Sanders said: "If it is correct that in the course of an interrogation, in which material supplied by the defendant [the British government] was employed, the claimant [Binyam Mohamed] was tortured, then it is arguable that there is an obligation to disclose material which may assist the claimant in establishing before the American military court that he was tortured."
Responding to the news, Clive Stafford Smith, director of Reprieve, said: "Mr Justice Sanders is clearly correct on the law, but let us rather consider morality.
"The British government cannot lend assistance to the torturers of the world -
whether knowingly, or merely with its head firmly in the sand - and then refuse to disclose the truth to the victim of the torture chamber."
The charges against Mr Mohamed, which could result in the death penalty, come despite earlier requests from British officials for his release.
David Miliband, the foreign secretary, made a formal request to his American counterpart Condoleezza Rice, in August 2006, asking for his release.
That request appears to have fallen on deaf ears, as have complaints from human rights groups about the legality of the trial itself, in which the defendant and his lawyers are denied access to some of the evidence against him.