An MI5 officer will not be prosecuted over claims he was complicit in the torture of former Guantanamo Bay detainee Binyam Mohamed.
Scotland Yard launched an inquiry after Mr Mohamed said an employee of the Security Service was aware of his ill-treatment while he was being held in Pakistan in 2002.
But director of public prosecutions Keir Starmer said there was "insufficient evidence" to prosecute the man, known as witness B, for any offence.
In a statement Mr Starmer said: "The Crown Prosecution Service has advised the Metropolitan Police that there is insufficient evidence to prosecute witness B for any criminal offence arising from the interview of Binyam Mohamed in Pakistan on 17 May 2002.
"We are unable to release further information at this stage because the wider investigation into other potential criminal conduct arising from allegations made by Mr Mohamed in interviews with the police is still ongoing."
The "wider investigation" is understood to refer to an inquiry into claims MI6 officials have also been linked to torture.
Detectives from Scotland Yard's specialist crime wing are examining "the conditions under which a non-Briton was held" and "potential involvement of British personnel".
A Metropolitan Police spokesman declined to comment. He said: "We will not give a running commentary."