“All of the work that Fusion GPS provided to The Free Beacon was based on public sources, and none of the work product that The Free Beacon received appears in the Steele dossier,” they said. “The Free Beacon had no knowledge of or connection to the Steele dossier, did not pay for the dossier, and never had contact with, knowledge of, or provided payment for any work performed by Christopher Steele.”

The Clinton campaign and the Democratic National Committee retained Fusion GPS to research any possible connections between Mr. Trump, his businesses, his campaign team and Russia, court filings revealed this week. Working for them, the firm retained Christopher Steele, a respected former British intelligence officer.

He went on to produce a series of memos that alleged a broad conspiracy between the Trump campaign and the Russian government to influence the 2016 presidential election on behalf of Mr. Trump. The memos, which became known as the “Steele dossier,” also contained unsubstantiated accounts of encounters between Mr. Trump and Russian prostitutes, as well as real estate deals that were intended as bribes.

Speculation had circulated for months that Mr. Singer was involved in the dossier, but his allies had denied that. On Friday, an associate said Mr. Singer had not been aware of the dossier or Mr. Steele’s involvement in Fusion GPS’s research until January, when BuzzFeed published the dossier.

Mr. Singer was among the leading Republican critics of Mr. Trump during the Republican presidential primary race. Even after Mr. Singer’s first choice, Senator Marco Rubio of Florida, dropped out of the race, Mr. Singer continued to fund efforts to block Mr. Trump from the party’s nomination.

The revelation of The Free Beacon’s involvement with Fusion GPS threatens a rapprochement between Mr. Trump and Mr. Singer, who donated $1 million to the president’s inaugural fund and has visited the White House on multiple occasions. After one such visit, Mr. Trump declared that Mr. Singer “was very much involved with the anti-Trump or, as they say, ‘Never Trump,’ and Paul just left, and he’s given us his total support and it’s all about unification.”