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An MI5 officer had confirmed the agent's status as he tried to persuade me to drop an investigation into X, a violent misogynist who used his Security Service role to coerce and terrify his former girlfriend, known publicly as "Beth".
Beth's lawyer, Kate Ellis from the Centre for Women's Justice, said the judgement was a "clear rejection" of MI5's explanations so far and a "serious warning" to the Security Service to cooperate.
She said the judges expressed concerns about "the ease" with which MI5 was able to "hide behind" the NCND policy - and warned that none of the safeguards to hold it accountable can work properly without "high standards of candour".
The two official inquiries criticised by the High Court were an internal MI5 inquiry and an "external" investigation by the government's former chief lawyer, Sir Jonathan Jones KC. The latter was commissioned by the home sectary and Sir Ken.
But the judgement said that "there was in our view a fundamental incoherence in Sir Jonathan's terms of reference".
Beth, pictured in a blurred silhouette against a high window, looking out onto tall buildings stretching into the distance on an overcast day
Image caption,
Beth has called for a public apology by MI5
The ruling said he was asked to establish the facts of what happened but not to "make findings about why specific individuals did or did not do certain things".
However, the judges said Sir Jonathan nevertheless "did make findings" that there was no deliberate attempt by anyone to mislead the court - without ever speaking to an MI5 officer at the centre of the case and without considering key additional BBC evidence about what took place.
The judgement also found that MI5's director general of strategy, who is the organisation's third-in-command, gave misleading assurances to the court in a witness statement.
He said its original explanations were "a fair and accurate account" of secret material which, at that point, had not been disclosed.
The court forced the government and MI5 to hand over the material, and the judges concluded that MI5's explanations were not "fair and accurate" and "omitted several critical matters" - including that IPCO had been misled and what was known by several MI5 officers at relevant times.
Their judgement said that it was "regrettable that MI5's explanations to this court were given in a piecemeal and unsatisfactory way - and only following the repeated intervention of the court".
"The impression has been created that the true circumstances in which false evidence came to be given have had to be extracted from, not volunteered by, MI5," they said.
X physically and sexually abused Beth, attacking her with a machete
Today's highly critical judgement also found:
In this one case MI5 has misled two separate branches of the High Court, as well as the Investigatory Powers Tribunal, the Investigatory Powers Commissioner, and security cleared barristers representing the BBC known as special advocates
MI5's core NCND secrecy policy about the status of agents was maintained in the legal proceedings long after "any justification for its maintenance had disappeared"
The BBC and I, as well as our lawyers and special advocates, should be "commended" for the "central role" we have played in bringing these matters to light
The judgement said that a "major" failing by the official reviews is that they did not contact me, despite the fact I was the other person involved in the key events.
The judges said that, having "considered carefully" further evidence I submitted in response to the reviews - such as records and notes that showed both reviews included false statements - it "paints a significantly different picture" to the one presented by MI5.
They added that they accepted the internal investigators and Sir Jonathan in the external review later considered my evidence "in good faith".
But they said that because they had already reached a conclusion that there had been no deliberate attempt to mislead the court, they would "inevitably find it difficult" to revise those conclusions in the light of evidence which "fundamentally affects" the basis of their conclusions.