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A controversial policy which allows MI5 to authorise informants to commit serious criminal offences - potentially including murder, kidnap and torture - is lawful, a tribunal has ruled.

But the Investigatory Powers Tribunal (IPT) said "this does not mean that (MI5) has any power to confer immunity from liability under either the criminal law or the civil law".

Four human rights organisations took legal action against the Government over a policy they claimed "purports to permit (MI5) agents to participate in crime" and effectively "immunises criminal conduct from prosecution".

Privacy International, Reprieve, the Committee on the Administration of Justice, and the Pat Finucane Centre asked the IPT to declare the policy unlawful and grant an injunction "restraining further unlawful conduct".

But, in a ruling on Friday, the tribunal ruled by a 3-2 majority that MI5, also known as the Security Service, does have the power to authorise the commission of criminal offences by informants.

In the majority ruling, IPT president Lord Justice Singh said MI5 has "an implied power" under the Security Service Act 1989 "to engage in the activities which are the subject of the policy under challenge".

But he added: "It is important to appreciate that this does not mean that it has any power to confer immunity from liability under either the criminal law or the civil law ... on either its own officers or on agents handled by them.

"It does not purport to confer any such immunity and has no power to do so."

Announcing the decision, Lord Justice Singh said: "This case raises one of the most profound issues which can face a democratic society governed by the rule of law."

Maya Foa, director of Reprieve, said: "The IPT's knife-edge judgment, with unprecedented published dissenting opinions, shows just how dubious the Government's secret policy is.

"Our security services play a vital role in keeping this country safe, but history has shown us time and again the need for proper oversight and common-sense limits on what agents can do in the public's name."

Ilia Siatitsa, a legal officer at Privacy International, said: "Today, the Investigatory Powers Tribunal decided that MI5 can secretly give informants permission to commit grave crimes in the UK, including violence.

"But two of its five members produced powerful dissenting opinions, seeking to uphold basic rule-of-law standards."

She added: "We think the bare majority of the IPT got it seriously wrong. We will seek permission to appeal to protect the public from this abusive secretive power."

Daniel Holder, deputy director of the Committee on the Administration of Justice, said: "The practice of paramilitary informant involvement in serious crime was a pattern of human rights violations that prolonged and exacerbated the Northern Ireland conflict.

"Archival documents show that the unlawful nature of informant conduct here was known at the time and it appears policy since has been even more formalised. This close ruling is far from the end of the matter."