In the first-ever dissenting opinions published in the tribunal’s 20-year history, two judges set out their disagreement with the 3-2 majority judgment.

One judge warned that the UK government’s claimed basis for the policy amounts to a “dangerous precedent”, while the other said the court had been asked to accept “fanciful” and “extraordinary” propositions.

The judgment was handed down today in the so-called ‘Third Direction’ case brought by four NGOs – Reprieve, the Pat Finucane Centre, Privacy International and CAJ – who argued that the policy has no legal basis and risked government involvement in severe rights abuses.

The NGOs, who launched their case in 2018, have confirmed their intention to challenge the IPT’s ruling at the Court of Appeal.

Maya Foa, director of Reprieve, said: “The IPT’s knife-edge judgement, 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, 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.

“As one of them put it, it is wrong to ‘open the door to… powers of which we have no notice or notion, creating uncertainty and a potential for abuse’. 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 CAJ, 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.”