LONDON—The award-winning journalist whose investigations led to the collapse of Donald Trump’s campaign-data gurus Cambridge Analytica and a record $5 billion fine for Facebook has launched a lawsuit for harassment against the man who bankrolled Brexit.

Carole Cadwalladr, a freelance investigative reporter, served the papers Monday against Arron Banks, the largest Brexit campaign donor. Lawyers acting on her behalf say a campaign of harassment, trolling, and threats of violence culminated Friday with a libel suit filed at the High Court against Cadwalladr for remarks she made during a TED talk, at a convention in London, and in a tweet.

“This is such an abuse of the law by Arron Banks. He’s not suing TED. He’s not suing The Observer or The Guardian. He’s a bully who’s targeting me as an individual to harass and intimidate me and prevent me from doing journalism, a course of behavior that has been going on for more than two years,” Cadwalladr told The Daily Beast.

Cadwalladr’s exposés of Cambridge Analytica and its alleged links to Banks and his Leave.EU campaign have appeared in The Observer and The Guardian newspapers. Multiple media outlets have also reported on Banks’ relationship with Russian officials, including the Kremlin’s ambassador in London. Banks and his team have not sued over those stories.

Britain’s National Crime Agency is investigating Banks for his involvement in the Brexit campaign, after the Electoral Commission said there were reasonable grounds to suspect Banks was “not the true source” of $10 million donated to Leave.EU.

Banks’ lawyers advised Cadwalladr two weeks ago that they were planning to take legal action over the TED talk, which has been viewed online more than two million times. In it, she said: “I’m not even going to go into the lies Arron Banks has told about his covert relationship with Russia.”

At an event called The Convention: Never Again held in June last year, she said: “We know that the Russian government offered money to Arron Banks.”

In both cases, Cadwalladr says she will use the defense that her statements were true. It has been reported that Banks was offered Russian mining contracts and Banks and his spokesman Andy Wigmore have repeatedly changed their stories on how many times they met with Russian officials in London.

“I think people can see exactly what’s going on here. It’s an attack on journalism by a man whose millions are now the subject of a criminal investigation. I can see why he’s touchy about me talking about his relationship with the Russian government, though. I would be too if I’d gone to the lengths he has to conceal it,” Cadwalladr said.

“What I really hope now is that members of Parliament will start asking hard and serious questions about why he twice visited the Russian embassy in the week he launched the Leave.EU campaign and why the Russian government might have wanted him to take one of the exciting gold and diamond deals it took great trouble to introduce him to.”

Cadwalladr’s solicitors wrote to Banks’ legal team Monday to say they would pursue a counterclaim if the libel action against her continues. “Litigation can form part of a course of conduct amounting to harassment,” her solicitor Ravi Naik wrote. “Should your client continue with these unmeritous proceedings, our client will counter claim against your client for harassment.”

Brexit campaigners have mocked Cadwalladr as a “crazy cat lady” and nicknamed her “Codswallop” after she persisted in pursuing the stories in print and via social media. Last week, Britain’s likely next prime minister, Boris Johnson, said a Cadwalladr story linking him with former White House chief strategist Steve Bannon was “codswallop.”

Wigmore told The Daily Beast that Cadwalladr was simply the first journalist to be targeted.

“It’s a very simple case—she has made statements in which she categorically said as fact Arron was offered money by the Russians and as fact Arron had received money from the Russians—she has to produce to a court evidence to support those comments. If she can, then she wins—if she can’t, she loses. There is no evidence because it didn’t happen—it’s that simple,” he wrote in an email.

“We have a series of other cases that will be placed in court over [the] next few days—including every single person/s organisation that’s have repeated those allegations as fact—remarkably quite a few, not ever using words to protect themselves—again they will all be asked to produce evidence to support reporting the allegations as fact. If they can then they win, if they can’t they lose and pay compensation etc.”

“FYI all winnings will be made as a donation to the Brexit party so some of these high-profile anti-Brexit supporters could conceivably become the most significant donors to the Brexit cause—oh the irony lol 😂”