Anti-Trump dossier writer Christopher Steele on Monday underwent his first adversarial questioning since his unverified charges were posted by BuzzFeed in January 2017.

Mr. Steele, a former British spy, has so far evaded requests by U.S. congressional committees to appear as a witness.

He is being sued for libel by Aleksej Gubarev, a Russian tech entrepreneur, who Mr. Steele said in his dossier was responsible for hacking Democratic Party computers and infecting them with spyware and pornography.

A British master ruled in March that Gubarev, who is also suing BuzzFeed, was entitled to question Mr. Steele within certain parameters.

The session happened Monday in London.

“I can confirm that the deposition took place today as scheduled, that Mr. Steele appeared, was professional and congenial and that we are satisfied with the results of the deposition,” Val Gurvits, a Gubarev attorney, told The Washington Times in an email. “I cannot, however, give you any details regarding Mr. Steele’s testimony because his entire testimony was designated as confidential by his legal team.”

Mr. Steele’s dossier has become one of the most important political documents in modern American politics and one that Trump associates deride as full of made-up collusion charges.

The dossier was based on unidentified Kremlin sources. It was funded by the Hillary Clinton campaign and the Democratic National Committee.

Fusion GPS, a Washington investigative firm, hired Mr. Steele and spread his dossier among friendly D.C. reporters at The New York Times, The Washington Post, Yahoo News and other sites.

Trump supporters say if the president’s critics are looking for Russian collusion in the 2016 election, they need look no further than the Democratic-financed dossier and its Russian disinformation.

None of Mr. Steele’s dossier collusion charges has been confirmed publicly.

The FBI relied on it greatly to obtain wiretaps and interview witnesses. Democrats in 2017 also repeatedly cited the dossier to bash President Trump, though their faith in Mr. Steele seems to have waned.

Fired FBI Director James Comey said his agents were given the task of trying to duplicate the dossier’s charges.

The Justice Department inspector general and a U.S. attorney in Utah are now investigating the FBI-dossier relationship.

Mr. Steele has been interviewed by special counsel Robert Mueller’s team.

Mr. Mueller has told the White House that Mr. Trump is not a target, a notification that apparently means the dossier is not true.

Mr. Steele said that Mr. Trump actively supported Kremlin computer hacking and worked with Russian intelligence for years. Mr. Trump has dismissed the charges as fiction.

Mr. Gubarev is suing Mr. Steele in a London court and BuzzFeed in Florida.

BuzzFeed has hired a consulting firm and its former FBI cybersleuth to try to prove the dossier is true.

Mr. Gurvits has said there is no evidence Mr. Gubarev played any role in Russian hacking of Democratic Party computers.

Mr. Gurvits told The Times that the deposition took four hours and was held in a private office.

There were more than a dozen people in the room: Mr. Gubarev’s two U.S. attorneys and his two British attorneys; two BuzzFeed attorneys and their two local counsel; Mr. Steele and his legal team; two representatives from the government, to make sure no state secrets came out; a court master; and a stenographer and a videographer.