The Secret Intelligence Service, MI6, has taken the unprecedented step of reporting one of its own officers to the attorney-general over allegations of complicity in torture.

The disclosure came in a letter from David Miliband, foreign secretary, to his Tory counterpart William Hague, published on Friday. The matter is being investigated by the Metropolitan Police, which is examining “the conditions under which a non-British citizen was detained and the potential involvement of British personnel”.