The revelation emerged in an ongoing legal case considering what crimes intelligence informants are allowed to commit, after it was revealed that MI5 maintains a secret policy under which agents can be “authorised” to commit offences. The case was brought by Reprieve, the Pat Finucane Centre, Privacy International, and the Committee on the Administration of Justice.

At a hearing, the tribunal concluded: “It was recognised that the direct communication that took place was inappropriate, an apology was given, and it was clearly recognised that nothing like this should happen in the future… and that something serious had gone wrong.” It declined to investigate further or seek additional disclosure from MI6.

MI6 sought to prevent the court from seeing secret inspection reports of the Investigatory Powers Commissioner, believed to detail MI6’s use of agents overseas. These were handed over to the tribunal by the commissioner.

The MI6 officers told tribunal staff that documents “should not have been provided” to them, and asked for the material to be returned to the agency and withheld from judges. After this attempt was discovered, the tribunal told MI6 it was “inappropriate for your staff to seek to intervene in ongoing legal proceedings in the way that they sought to do.”

A senior MI6 official was forced to apologise “for any misunderstanding” for the “approach” made to the tribunal, claiming that they had only sought “to understand better the nature of SIS information apparently referenced” in documents provided to it.

Maya Foa, Reprieve’s director, said: “Britain’s security services play a crucial role in keeping this country safe, but they do not get to decide what evidence a court should see. MI6 was right to apologise.”

Daniel Holder of CAJ and Paul O’Connor of the Pat Finucane Centre, said: “There has long been a long concerning history in Northern Ireland of the intelligence services considering themselves not bound by the law and above and beyond its reach. Such practices are incompatible with the principles of a democratic society and go to the heart of this case. Its therefore particularly alarming that MI6 sought to interfere with the tribunal in this way.”

Ilia Siatitsa, legal officer and programme director, Privacy International, said: “Such an interference with judicial proceedings has absolutely no place in any mature democracy. In another PI case before the same tribunal in 2017, it was revealed that GCHQ had also made advances of similar nature to the Commissioner. It is troubling the Agencies have not yet learned these basic principles.”