British police are to investigate whether the country's intelligence agency was complicit in the alleged torture of a former Guantanamo detainee.
Patricia Scotland, the attorney-general, said on Thursday she had ordered police to begin an inquiry into claims by Binyam Mohamed that an officer from MI5 knew he had been tortured.
She told politicians the inquiry would be conducted "as expeditiously as possible given the seriousness and sensitivity of the issues involved".
Mohamed, 30, has said he was tortured in custody in Pakistan and Morocco, before being transferred to the Guantanamo Bay prison camp in 2004.
He says that an MI5 agent provided questions that he was asked by interrogators who tortured him at a secret site in Morocco.
Zachary Katznelson from Reprieve, the legal charity which represents Mohamed, welcomed Scotland's announcement.
"The attorney-general absolutely did the right thing today. It is critical that we get to the bottom of what was done to Binyam Mohamed and the role of any British official in his torture," he said.
But he added that police must be given access to secret information for it to be a "proper inquiry".
Ethiopian-born Mohamed was released from the Guantanamo Bay detention centre in February, after spending seven years in US captivity without charge.
The decision to release him followed a formal request by London to Washington in August 2007, asking for the remaining five British residents held at the camp on Cuba to be released.
Mohamed was arrested in Pakistan in 2002 on suspicion of attending an al-Qaeda training camp in Afghanistan and plotting to build a radioactive "dirty" bomb.
Speaking after his arrival into Britain, he claimed he was tortured and abused by foreign agents, saying: "I have been through an experience that I never thought to encounter in my darkest nightmares.
"It is difficult for me to believe that I was abducted, hauled from one country to the next, and tortured in medieval ways, all orchestrated by the United States government."