Former President Donald J. Trump is arguing that the document known as the Steele dossier was calculated to embarrass him and that it breached data protection laws.Donald J. Trump has claimed in a lawsuit in a London court that Christopher Steele, a former British intelligence officer, inflicted “personal and reputational damage and distress” on him by leaking a dossier detailing unsavory, unproven accounts of links between him and Russia during the 2016 presidential campaign.Lawyers for Mr. Trump argue that Mr. Steele’s firm, Orbis Business Intelligence, breached British data protection laws with the dossier, which triggered a political earthquake when it was published just before Mr. Trump’s inauguration in 2017.The lawsuit, the first filed by Mr. Trump in Britain related to the dossier, could offer the former president more favorable legal terrain than the United States. Last year, a federal judge in Florida threw out his lawsuit claiming that Mr. Steele, as well as Hillary Clinton and the Democratic National Committee, was involved in a concerted plot to spread false information about Mr. Trump’s ties to Russia.In a court filing last month, Mr. Trump’s lawyers said he was “compelled to explain to his family, friends, and colleagues that the embarrassing allegations about his private life were untrue. This was extremely distressing” for him, the filing said, asserting that Mr. Steele had presented the claims in a “sensationalist manner” that was “calculated to cause tremendous embarrassment” to Mr. Trump. He is asking for unspecified compensation.The High Court judge Matthew Nicklin has scheduled a two-day hearing on Oct. 16 and 17, at which arguments will be heard and lawyers for Mr. Steele’s firm will move to throw out the case, which was originally filed last November.The dossier’s author, Christopher Steele, center, in 2020. He has accused Mr. Trump of engaging in “frivolous and abusive legal proceedings” in the United States.Victoria Jones/Press Association, via Associated PressIn a witness statement, Mr. Steele accused Mr. Trump of “numerous public attacks upon me and Orbis.” He said the former president had initiated “frivolous and abusive legal proceedings” against him and his firm in the United States, a conclusion echoed by the Florida judge’s ruling.A spokesman for Mr. Trump did not respond to requests for comment, and neither did his British lawyers, while Mr. Steele declined to comment.Mr. Trump’s foray into the British courts comes as he is facing a raft of criminal and civil charges in the United States, on accusations ranging from election interference to inflating the value of his real estate assets — all of which he has denied. He has experienced a string of legal setbacks in courtrooms from Manhattan to South Florida.But in London, Mr. Trump is the plaintiff, and legal experts said his lawyers were trying to seize an advantage from Britain’s comparatively tight controls on personal data. Winning a claim that his data had been compromised, these lawyers said, would be easier than winning a claim of defamation.“It avoids the obvious hurdles of a U.K. defamation claim,” said Jay Joshi, a media lawyer with the London firm Taylor Hampton. These include the statute of limitations for defamation, normally a year, and the fact that the dossier was published in the United States, not Britain. “Trump is clearly seeking some form of vindication,” Mr. Joshi said.In 2020, Aleksej Gubarev, a Russian technology entrepreneur who was cited in the dossier, lost a defamation suit against Mr. Steele. But in another case that year, two Russian oligarchs, Mikhail Fridman and Petr Aven, won damages of 18,000 pounds ($22,900) each from Mr. Steele’s firm after they argued that allegations about them in the dossier violated data-protection laws.The court ruled that Orbis had “failed to take reasonable steps to verify” claims that Mr. Fridman and Mr. Aven, who controlled Alfa Bank, had made illicit payments to President Vladimir V. Putin of Russia, though the judge dismissed several other claims.Mr. Trump’s lawyers are making a similar claim that Mr. Steele’s firm did not confirm the claims about him. Among other things, they said, Mr. Trump did not bribe Russian officials to advance his business interests.“The claimant did not engage in unorthodox behavior in Russia and did not act in a way that Russia authorities were provided with material to blackmail him,” the lawyers said. “The personal data is not accurate. Further, the Defendant failed to take all reasonable steps to insure the personal data was accurate.”Mr. Trump is being represented by Hugh Tomlinson, a leading London media lawyer who specializes in defamation, privacy and data protection. Among his former clients is King Charles III, then the Prince of Wales, for whom Mr. Tomlinson argued successfully that a British tabloid should not be allowed to publish his private diaries, which contained astringent comments about the 1997 handover of Hong Kong to China.The Steele dossier grew out of an opposition research effort to dig up information about Mr. Trump, funded by Mrs. Clinton’s campaign and the Democratic Party. Their law firm, Perkins Coie, contracted with a Washington research firm, Fusion GPS, which in turn hired Mr. Steele, an expert on Russia, to research Mr. Trump’s business dealings in the country.Mr. Steele shared some of the memos with the F.B.I. and journalists; they first came to light in January 2017 when Buzzfeed published 35 pages.His findings have been largely discredited by the F.B.I. and others who have investigated Mr. Trump’s relationship to Russia. Relying on anonymous sources, the dossier asserted that there was a “well-developed conspiracy of coordination” between the Trump campaign and the Russian government, and that Russian officials had a blackmail tape of Mr. Trump with prostitutes.For much of his information, Mr. Steele relied on Igor Danchenko, a Russian researcher who told federal investigators that some of the claims were rumors that he had not been able to confirm. Mr. Danchenko was later indicted on a charge of misleading federal investigators, but he was ultimately acquitted.The F.B.I. concluded that one of the most explosive allegations in the dossier — that Mr. Trump’s lawyer, Michael Cohen, had met with Russian officials in Prague during the 2016 campaign — was false.In his witness statement, Mr. Steele said he wrote the memos on a computer that was not connected to a network and was equipped with security that prohibited any third party from extracting data stored on it. He also said that Orbis no longer held any copy of the dossier on its systems by the end of the first week of January 2017.Mr. Steele has not denied sharing the dossier with journalists. But he rejected the contention that he has sought to promote its contents since then.“I declined to provide any media interviews for three and a half years after the publication of the dossier by Buzzfeed, despite being asked multiple times by major international media organizations,” he testified. “If I had wanted to ‘promote’ the dossier as Mr. Trump suggests, I obviously would have taken up those media opportunities.” More