The broadcaster argued that MI5 and three of its officers had met the legal threshold for contempt of court, having provided false evidence in a bid to protect the agent’s identity.

Sir James Eadie KC, representing the Security Service, issued an “unreserved apology” but maintained that “the errors that had been made had not been deliberate”.

Judgement was reserved by Lady Chief Justice Baroness Sue Carr, Dame Victoria Sharp, and Mr Justice Chamberlain.

The case dates back to 2022, when MI5 attempted to block a BBC investigation into a violent far-right agent known only as X. It has since become a landmark test of judicial oversight of the Security Service.

MI5 gave evidence to three separate courts insisting it had never breached its ‘neither confirm nor deny’ (NCND) policy regarding the agent’s status. However, in February the BBC presented recordings and notes of calls with MI5 contradicting this claim.

An officer had confirmed X’s role in the service during an attempt to persuade a BBC journalist to drop the investigation. The agent, a neo-Nazi with a history of violent misogyny, was alleged to have used his MI5 affiliation to intimidate his former partner, known publicly as Beth.

At Tuesday’s hearing, MI5 admitted that its NCND stance could no longer be upheld in this case.

Due to that policy, large portions of evidence had previously been heard in closed sessions, excluding both Beth and the BBC from proceedings. Beth is pursuing her own legal action against the service.

Her solicitor, Kate Ellis, welcomed the outcome, saying: “It’s a huge outcome for Beth. She’s had to fight this all the way to the High Court. This case has really undermined MI5’s credibility in the courts.”