The man responsible for overseeing the conduct of the UK's intelligence services has said that MI5 lying to multiple courts while defending a violent neo-Nazi spy is as "serious as anything" he has ever dealt with.

"Oversight depends on candour," said investigatory powers commissioner Lord Leveson. "Failings of candour undermine the entire basis of oversight and accountability."

His office, the Investigatory Powers Commissioner's Office (IPCO), investigated how MI5 gave false evidence to three courts concerning the abusive informant known as agent X, as first exposed by the BBC.

IPCO's report, released earlier this year, found one senior officer lied repeatedly while another misled his own colleagues and lied about what he was told.

It also criticised other senior spies and found "serious and systemic failures in MI5's conduct" throughout the case.

MI5 said it accepted the report's findings "without reservation" last month. The High Court is due to decide soon on what action, if any, to take against the agency or its staff.

In comments to mark the publication of his annual report on Thursday, Lord Leveson said the "case of agent X is as serious as anything I have dealt with as commissioner".

"False evidence was given to the High Court and the Investigatory Powers Tribunal, and the prime minister directed me to find out how that happened.

"My investigation has now concluded and found that both IPCO and the courts were misled."

The IPCO investigation examined how MI5 gave false evidence to three courts about having kept to its core secrecy policy - known as 'neither confirm nor deny' (NCND) - about the agent status of the violent neo-Nazi informant.

He coercively controlled his partner, known by the alias Beth, and attacked her with a machete. MI5 then helped him go abroad to continue intelligence work while he was under police investigation.

The government took the BBC to court in 2022 in a failed attempt to block our investigation into X, but won him legal anonymity. Beth then sued MI5 at the Investigatory Powers Tribunal, and in 2024 she sought a review of a ruling at the High Court.

MI5 claimed it had maintained NCND and, consequently, the courts allowed it to keep information secret from Beth and everyone else, leaving her at a serious disadvantage.

But what MI5 said was untrue. It had in fact disclosed the man's agent status in phone calls to me, as it tried to persuade me not to investigate him in 2020.

A senior MI5 officer had tried to cover up for the man, falsely saying he was not an abusive misogynist nor a real extremist. He even wanted me to meet the agent. In doing so, he repeatedly departed from the NCND policy.

That officer was found to have repeatedly lied by the IPCO investigation. He has resigned from MI5 since the BBC exposed what happened.