The Prime Minister is facing calls to reveal the guidance on when UK agents can commit criminal offences and how far they can go.

Theresa May
Image: Theresa May revealed the secret function in a written statement to Parliament

Human rights campaigners have demanded the Government reveal the rules for British spies in committing criminal offences, after the Prime Minister admitted such guidance exists.

In a written statement to Parliament on Wednesday, Theresa May made a rare reference to UK security service agents potentially participating in criminality.

It followed a legal battle by campaigners for the Government to disclose the oversight functions of the Investigatory Powers Commissioner (IPC), Lord Justice Fulford.

Two of three oversight functions, relating to the collection of personal data and the rules for detaining people overseas, had previously been made public.

But a third secret function, referred to as the "third direction", has now only been revealed.

In her statement, the Prime Minister said: "The other direction instructed the Commissioner to keep under review the application of the Security Service guidelines on the use of agents who participate in criminality and the authorisations issued in accordance with them."

Millie Graham Wood, a solicitor at Privacy International, who battled for the disclosure of the previously secret function alongside human rights organisation Reprieve, said: "There is no justification why this secret direction, whose existence was revealed as a result of Privacy International's litigation, was not published earlier.

"Had we not sought to challenge the Government over the failure to publish this direction, together with Reprieve, it is questionable whether it would have ever been brought to light.

"It is wrong in principle for there to be entire areas of intelligence oversight and potentially of intelligence activity, about which the public knows nothing at all."

The Prime Minister's reference to potential agent criminality also prompted calls for her to now publish the guidance overseen by the IPC.

Maya Foa, Reprieve's director, said: "After a seven-month legal battle the Prime Minister has finally been forced to publish her secret order but we are a long way from having transparency.

"The public and Parliament are still being denied the guidance that says when British spies can commit criminal offences and how far they can go.

"Authorised criminality is the most intrusive power a state can wield. Theresa May must publish this guidance without delay."

In order to step up pressure on the Government to publish the guidance, Reprieve are now launching a campaigning video, including actor Benedict Cumberbatch, featuring more than 50 people acting out what activity they think agents could be involved in.