Human rights group Liberty joined forces with its American counterpart to demand a public inquiry into allegations that a terror suspect has been secretly detained in a British territory.
The activists wrote to Foreign Secretary David Miliband urging him to launch a formal investigation into claims about US activities on Diego Garcia, the UK territory in the Indian Ocean.
Last month the news magazine Time published allegations by a former senior US official that at least one terror suspect was imprisoned on the island in a secret jail, known as a "black site".
The island had already been implicated as a "staging post" in secret flights transporting suspects from one country to another, as part of so-called extraordinary rendition measures.
The letter to Mr Miliband marked the first such collaboration between Liberty and the American Civil Liberties Union (ACLU).
Liberty's director Shami Chakrabarti said: "We will not rest without real answers to the scandal of torture flights and inhuman interrogation in freedom's name."
ACLU executive director Anthony Romero added: "The ACLU is proud to join Liberty in the effort to uncover the truth about what happened at Diego Garcia.
"We are convinced that an official inquiry with sufficient investigatory authority is the only way to restore confidence in our commitment to human rights and repair the damage done to the reputations of our nations as a result of unlawful detention policies.
"Rendition flights and brutal interrogation practices have no place in a functioning democracy."
The letter to the Foreign Secretary said the use of UK territory in secret, illegal operations would "bring our international reputations into disrepute".