MI5 agents have secretly been given authorisation to participate in ‘murder, torture and sexual assault’ on British soil without fear of prosecution, a tribunal heard yesterday.

It emerged that the security service has been giving its informants and agents the freedom to commit ‘grave criminality’ for almost 30 years.

Yet no police officer or prosecutor has ever been told of their criminal activities, the Investigatory Powers Tribunal in London heard.

A secret letter from former prime minister David Cameron was made public yesterday. This effectively gave MI5 agents a licence to kill, campaigners claim

A secret letter from former prime minister David Cameron was made public yesterday. This effectively gave MI5 agents a licence to kill, campaigners claim.

The bombshell document emerged during a legal challenge by privacy campaigners, who want know what crimes have been committed in the name of MI5 since the 1990s and whether they were lawful.

In November 2012, Mr Cameron wrote to retired judge Sir Mark Waller acknowledging that there was a ‘long-standing’ secret policy to let security service agents break the law.

He instructed Sir Mark, who was at the time the Intelligence Services Commissioner, charged with overseeing the conduct of MI5, MI6 and GCHQ, to have oversight of the policy.

The bombshell document emerged during a legal challenge by privacy campaigners, who want know what crimes have been committed in the name of MI5 since the 1990s and whether they were lawfu

In November 2012, Mr Cameron wrote to retired judge Sir Mark Waller acknowledging that there was a ‘long-standing’ secret policy to let security service agents break the law

But the then-prime minister told him not to rule on whether it was legal, and said he need not express any views as to whether any cases should be referred to prosecutors.

Privacy campaigners claim the letter effectively handed MI5 agents a licence to break the law with immunity.

The timing of the letter is said to be highly significant as just two weeks later Mr Cameron admitted there was ‘state collusion’ in the 1989 murder of Belfast solicitor Patrick Finucane.

Mr Finucane, who represented several high-profile Republicans, was shot dead in front of his family by loyalist gunmen. After his death it emerged that the loyalist paramilitary intelligence officer responsible for directing Ulster Defence Association attacks, Brian Nelson, was an agent controlled by the British Army’s ‘Force Research Unit’. No one has been prosecuted for the murder.

But the then-prime minister told him not to rule on whether it was legal, and said he need not express any views as to whether any cases should be referred to prosecutors

Privacy campaigners claim the letter effectively handed MI5 agents a licence to break the law with immunity

Mr Cameron wrote in the newly disclosed letter: ‘In the discharge of their function to protect national security, the security service has a long-standing policy for their agent handlers to agree to agents participating in crime, in circumstances where it is considered such involvement is necessary and proportionate in providing or maintaining access to intelligence that would allow the disruption of more serious crimes or threats to national security.’

The timing of the letter is said to be highly significant as just two weeks later Mr Cameron admitted there was ‘state collusion’ in the 1989 murder of Belfast solicitor Patrick Finucane

Official MI5 guidance entitled ‘guidelines on the use of agents who participate in criminality’ was also made public yesterday for the first time. The policy states that an officer is ‘empowered’ to ‘authorise the use of an agent participating in crime’.

Ben Jaffey QC, representing an alliance of human rights groups, told the tribunal that Mr Cameron’s letter demonstrated that no police or prosecutor would ever hear about the cases involved.

Sir James Eadie QC, representing the intelligence agencies, the Home Office and the Foreign Office, told the tribunal that details of MI5’s conduct had to be kept secret and he asked that the hearing go into private to hear his reasons.

Reprieve director Maya Foa said: ‘We want to know if it’s government policy to let MI5 agents get away with serious crimes such as torture and murder.

‘While our intelligence agencies have an important role in keeping this country safe, it does not follow that agents can be permitted to break the law without limits.’