Spies lie, but they are not meant to get caught.

On a wet Friday evening in December, three MI5 lawyers sat in a room at the BBC's headquarters in London. On the other side of the table were the BBC's lawyers and me. No MI5 officers were present, after we refused a request for the meeting to be secret.

After exchanging strained smiles, we got down to business - and proved to them that the Security Service had been giving false evidence to the courts.

The meeting took place after I told MI5 in November that we were planning to report it had lied and offered it a chance to comment. In response, the Security Service insisted - aggressively so - that it had been entirely honest.

What it hadn't realised until the December meeting, was that I had hard evidence to prove its position was false.

The revelation of the false evidence matters because it raises serious concerns about how reliable MI5's evidence is in the courts, where assessments from the Security Service are given enormous deference.

It also raises fresh doubts about whether MI5 can continue with a core policy of secrecy - after we revealed it was applying it selectively.

The organisation's first lie came when the government took the BBC to the High Court in 2022 to block a story about a right-wing extremist working as an MI5 agent - the term for a paid and authorised informant.

Former Attorney General Suella Braverman failed to prevent us publishing the story but succeeded in getting an order preventing the man from being identified, having argued he would be in danger. As a result, the man is known publicly only as X.

We argued he should be identified so women could be warned about such a predatory and violent man. X had used his MI5 role to coercively control his ex-partner, known publicly by the alias Beth. He was physically and sexually abusive, and was filmed threatening to kill her and then attacking her with a machete.

During the legal proceedings, MI5 said it could publicly neither confirm nor deny (NCND) whether X was an agent, in line with its long-standing policy.

In public, MI5 emphasises the fundamental role of the NCND policy, which it says it follows in relation to its agents. Any exceptions are said to undermine the secrecy that protects such men and women and to damage national security.

But then, during the proceedings, a senior MI5 officer - a deputy director of the organisation and senior counter-terrorism officer, known in court as Witness A - mentioned some phone calls involving me.

In a corporate witness statement, Witness A said an MI5 representative had spoken to me at an earlier date. This came after the Security Service became aware the BBC intended to include X in an investigative story.

Witness A said that, during the discussions, I said I had suspected X was a state agent, but that MI5 had "neither confirmed nor denied" whether this was the case.

He did not explain who initiated the discussions - it was MI5. Nor did he say how the Security Service had become aware of the planned BBC story - X had told them.

In my own detailed witness statement, I said this account did not correspond with my recollection in various respects. I did not go into further detail, because - without any waiver of legal privilege - I had been advised by lawyers it would not affect the key issues in the dispute, which centred on the claimed risks to X if he were identified by the BBC.

The BBC team also only had access to parts of the case - some evidence and hearings were in secret.

During one secret part of the proceedings, to which the BBC team and I did not have access, security-cleared lawyers appointed on our behalf were also told by MI5's lawyers that it had maintained its neither confirm nor deny policy in the calls with me.

But I knew that what Witness A said about me in his statement was untrue. I knew that MI5 had not abided by NCND. To be honest, I resented its lie.

We can also now report that MI5 director general Sir Ken McCallum called the BBC director general Tim Davie, in December 2021, to cast doubt on the BBC's planned story about X.

Sir Ken's own notes of the calls, which were served as part of the government's evidence in the 2022 High Court case, record him claiming the intended story was "inaccurate as well as reckless".

However, the story was accurate. The judge found I had taken proper steps to assess whether the story's various elements were true and that it was "comfortably" shown to have a credible evidential foundation.

Despite not being allowed to identify X, the High Court ruled we could report the results of our investigation. It showed that X had abused Beth and another former partner, and that he was a dangerous extremist.

At about the same time, Beth lodged a formal complaint about her treatment by MI5 with the Investigatory Powers Tribunal (IPT), an independent court with the power to investigate human rights claims against the Security Service.

It was at this court where MI5 would lie again.

The IPT often sits in secret without claimants, their legal representatives, journalists or the public present, to consider evidence deemed sensitive for national security reasons. In the case of X, this meant all the information which confirmed he was an MI5 agent.

Lawyers for the IPT are meant to represent the interests of claimants in the........

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