Unfounded claims that Mr Trump had been filmed with prostitutes in a Moscow hotel surfaced in the closing stretch of last year's White House race.

Mrs Clinton's presidential campaign and the Democratic National Committee (DNC) reportedly helped fund the research.

"The victim here is the President," Mr Trump tweeted on Wednesday.

According to US media reports, Perkins Coie, a law firm representing the Clinton campaign and DNC, hired intelligence firm Fusion GPS in April 2016.

Fusion GPS, based in Washington DC, was paid to dig up dirt on Mr Trump, who was then Mrs Clinton's rival for the presidency.

The intelligence firm subcontracted Christopher Steele, a former British spy who previously worked in Russia, to compile the research.

The opposition research was initially funded by an unknown Republican consulting firm, which pulled the plug once Mr Trump captured the party's nomination.

The Clinton campaign then picked up the tab, according to the reports.

As he headed off to Dallas, Texas, on Wednesday, President Trump told reporters on the White House lawn: "It's very sad what they've done with this fake dossier.

"It was made up and I understand they paid a tremendous amount of money."

He added: "Hillary Clinton always denied it, the Democrats always denied it.

"I think it's a disgrace, it's a very sad commentary on politics in this country."

He dismissed the dossier shortly before his inauguration as "fake news".