Jacqui Smith, the Home Secretary, has written to the Attorney-General to ask her to investigate “possible criminal wrongdoing” by MI5 and the CIA in relation to a British resident held in Guantanamo Bay.

The action, which is believed to be without precedent, arose after a High Court judgement in September found that MI5 may have been complicit in an unlawful interrogation of Binyam Mohamed, an Ethiopian who has been granted resident status in Britain.

Baroness Scotland, the attorney general, will now invesitgate evidence, which was heard in closed court but has not been made public.

An MI5 officer interrogated Mr Mohamed while he was being held in Pakistan in 2002. The Security Service also questioned all the British prisoners held at Guatanamo Bay by the Americans, although it was always on the basis that nothing that was said could be used in any future prosecution.

The objective of the interrogation was to discover whether the British detainees knew of anything which might have a direct relevance to Britain’s national security interests. It is believed that the questioning of Binyam Mohamed was carried out on the same basis, with the approval of the CIA.

Security sources last night would not comment on the case of Binyam Mohamed other than to say: “This matter has been referred to the Attorney-General and now there is a legal process.”

Whitehall sources said that the Home Secretary was obliged to take the matter further because of the judge’s comments.

Clive Stafford-Smith, who represents British detainees at Guantanamo Bay in his role as director of the legal charity Reprieve, suggested that the action represented a recognition that Mr Mohamed had been treated unlawfully.

“This is a welcome recognition that the CIA cannot just go rendering British residents to secret torture chambers without any consequences, and British agents cannot take part in American crimes without facing the music,” he said. “Reprieve will be making submissions to the Attorney-General to ensure that those involved in these crimes — from the US, Pakistan, Morocco, Britain and elsewhere — are held responsible.”

Mr Mohamed has alleged that, while being held in Morocco, he was tortured by having his penis cut with a razor blade.

He claims that all evidence against him was produced under torture and has sought to force MI5 to disclose material that would corroborate his claims.

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