An agent working for MI5 with a background in rightwing extremism abused his former partner and used his connection with the domestic intelligence agency to threaten her further, according to an investigation by the BBC.

The man – known only as X for legal reasons – is said to have terrorised the woman and at one point attacked her with a machete and threatened to kill her, as shown in a video captured on her mobile phone. “There was so much psychological terror from him to me, that ultimately culminated in me having a breakdown,” the woman said.

The BBC was only able to tell her story after winning a high court battle with the government. Suella Braverman, the attorney general, had applied for an injunction preventing X from being identified, while the BBC argued it was in the public interest to name a man who allegedly poses an ongoing threat to women.

In the end the BBC was blocked from identifying the man, a foreign national who worked as a paid informant for MI5 infiltrating extremists networks in the UK. But it was able to broadcast a film about him, released on Thursday.

MI5 has a policy of never confirming the identity of its agents or paid informants, although Braverman’s case was mounted on the “hypothetical assumption” that X was an agent. He has reportedly left the UK, and went on to work for a foreign intelligence agency.

His former partner, a Briton who the BBC has called Beth to protect her identity, said X told her she would not be able to report his abusive behaviour because of his work for MI5.

“It meant that I couldn’t speak out about any of his behaviour towards me, any of the violence I went through, sexual or physical, because he had men in high places who always had his back, who would intervene and who would actively kill me, if I spoke out,” Beth said.

During their time together, Beth said she was aware that X was having meetings with contacts, receiving payment and being given surveillance equipment.

Although X was paid to inform on networks of rightwing extremists, Beth said he appeared to share their beliefs. The informant praised white supremacist mass murderers and even said that he wanted to commit a similar act of violence.

Police did investigate the machete attack, during which X allegedly said he would kill Beth, but the Crown Prosecution Service (CPS) dropped the case as it came to court. Beth said the police did not take a full statement from or obtain the mobile video of the violent incident, where X tried to attack her with both the large knife and his fists. The police force and the CPS said the case was discontinued for lack of evidence.

X returned to their home and Beth said he continued to threaten her, claiming the security services would support him and that she was “nothing”. But gradually their relationship collapsed.

X became subject of another police investigation, which drew in counter-terrorism officers, after it emerged that he had had a private diary in which he allegedly wrote about killing Jews and kept Nazi materials. However, X left the country before the inquiry concluded.

A second woman, who had a relationship with X in the same foreign country before he came to the UK, also told the BBC that X was violent to her. The woman, who was given the alias Ruth, said: “He said he would be able to kill me and my daughter, too, and then put our bodies somewhere and no one would ever know who I am.”

Ruth had not met or been aware of Beth. She eventually fled to a refuge to escape X and said she was “psychologically broken” by their relationship. During their time together she remembered X kept a book contained lurid fantasies involving “eating children’s flesh”.

The BBC said it had established X worked for MI5 as a paid informant for years using various false names. “This is the story of a dangerous MI5 agent, which the government tried to keep secret,” it added.

In high court rulings, Mr Justice Chamberlain had said that alleging that X was an undercover informant would put his life at risk. He said the “experience and expertise” of the corporation meant it could make decisions about which details risked identifying X.