Once secret guidance on MI5's use of agents who engage in crime published by Theresa May

Disclosure represents an extremely rare official reference to the potential involvement in crime of agents

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Theresa May has published a previously secret direction relating to MI5's use of agents who participate in crime.

The Prime Minister confirmed that the area of the Security Service's work was kept under review by a watchdog.

In a written statement to Parliament, Ms May referred to a direction which instructed the Investigatory Powers 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".

The disclosure represented an extremely rare official reference to the potential involvement in crime of agents run by MI5.

Human rights organisation Reprieve said it was the first time the Government had acknowledged that guidance existed within the UK to regulate such activity.

Investigatory Powers Commissioner Lord Justice Fulford said he welcomed the Government's decision to "make public my oversight of this sensitive area of work".

The oversight was previously conducted by the Intelligence Services Commissioner - one of three bodies whose work was taken over by the Investigatory Powers Commissioner's office last year.

A section of a report published by Intelligence Services Commissioner Sir Mark Waller in 2016 was headed "agent participation in the commission of an offence".

It said: "There may be occasions where a CHIS (Covert Human Intelligence Source) participates in a criminal offence in order to gather the required intelligence, for example membership of a proscribed organisation or handling stolen goods.

"However in specific situations where the intelligence dividend justifies it, a good argument can be made that it is in the public interest and for the greater good to become involved.

"Although such activity cannot be made lawful I have recommended that the agency must justify the public interest test."

MI5 describes its agents as "one of the most significant information gathering assets we have".

Agent operations are run by specially trained officers, or handlers, and can continue for long periods, sometimes for many years, according to the domestic intelligence agency's website.

In a speech last year, MI5 director general Andrew Parker said: "I want to record special thanks to our agents, sometimes known as human sources. Those who courageously work for us in secret, close to the extremists, who do so much to help us prevent terrorist atrocities.

"We all owe them a debt of gratitude."

Maya Foa, director of Reprieve, called on the Government to publish the guidelines referenced in Mrs May's statement. She said: "Authorised criminality is the most intrusive power a state can wield."