Lord Leveson said the BBC's investigation had "hit the intelligence community hard"

The man responsible for overseeing the conduct of MI5 has said it will now have to provide proof to be believed by judges, after a BBC News investigation exposed how the security service lied to three courts.

Lord Leveson, the investigatory powers commissioner and a former Court of Appeal justice, said that as a judge "if the security service said something to me, then I would accept it".

But he said courts would now be "far more reluctant to accept, without evidential proof, things that are said in the future".

Lord Leveson praised the impact of the BBC's investigation into an abusive MI5 agent, which he said had led to changes in how informants are managed.

He spoke to the BBC during the first annual conference of the Investigatory Powers Commissioner's Office (Ipco).

The BBC's investigation had "hit the intelligence community hard", including security services around the world, he said.

The BBC investigation exposed how a neo-Nazi informant - known as Agent X - used his state role as a tool of abuse. He coercively controlled his partner, known by the alias Beth, and attacked her with a machete. He had previously abused another woman and threatened to kill her child.

Despite all this, MI5 protected the agent and helped him go abroad to continue intelligence work while he was under police investigation, even removing his possessions from a police station.

MI5 agent used secret status to terrorise girlfriend

How I exposed MI5's lie about its violent abusive agent

Following the BBC story, a secret inquiry by Ipco concluded the security service knew the agent was a misogynist who was "obsessed" with violence.

Ipco said MI5 should have reassessed his suitability as an agent.

At the request of the then Prime Minister Sir Keir Starmer, Ipco also investigated how MI5 gave false evidence to three courts concerning Agent X, something exposed by the BBC.

Ipco's report, released earlier this year, found one senior MI5 officer had lied repeatedly while another had 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 has apologised and said it accepted the report's findings "without reservation". The High Court has declined to launch contempt of court proceedings, and said the police might be better placed to take on the case.

Agent X had used his status to coercively control his then girlfriend, including attacking her with a machete

In his first interview about the case, Lord Leveson said he stood ready to assist the police if asked.

He said that "MI5 have a great distance to go" with courts in the future.

He added the BBC investigation's reach "has been far greater than if this had never happened. So you can take some pride in that."

Lord Leveson said there was also a change in the way that agents, also known as covert human intelligence sources (CHIS), were managed.

Because of the Agent X case, there is a new policy relating to agents and domestic violence, and coercive control.

"There is no doubt that concern is now being focused not merely on the activity of the CHIS, but the impact of the CHIS on other aspects of his or her life.

"I think that everybody learns from disasters, and this for MI5 has been a very serious matter."

The Ipco chief executive, Richard Thompson, now attends meetings of an internal board in MI5, created to oversee changes because of the Agent X case, including changes in culture within the service.

In addition, MI5 must change the way it prepares and gives evidence, after the High Court case.

Lord Leveson said intelligence service chiefs from around the world had contacted him about the case.

They had expressed concern about whether something similar could happen in their organisations, he said.

The Agent X case had "done a great service to everybody", Lord Leveson said

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.

The government took the BBC to court in 2022 in a failed attempt to block our investigation into Agent X, but won him legal anonymity. His ex-partner 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.