A leaked intelligence 'dossier' that claims Donald Trump is being blackmailed by the Russians came from a corporate espionage firm in Washington that Democrats, Republicans and private companies have relied on for years to dig up dirt on competitors.

Despite the secretive nature of its work, Fusion GPS has been at the center of a number of political controversies.

The secretive firm, which is run by three former Wall Street Journal reporters, was hired by Planned Parenthood to push back against undercover videos that appeared to show officials selling fetal tissue.

The firm also worked for Democrats in 2012 to uncover dirt on one of Mitt Romney's top donors, leading to charges from conservatives that the Obama campaign was engaging in 'Nixonian' tactics.

The Fusion GPS intelligence firm, which was co-founded by former Wall Street Journal reporter Glenn Simpson (pictured), was first hired by a Republican donor in 2015 to dig up dirt on Trump

Peter Fritsch (left) and Thomas Catan (right), both former reporters at the Wall Street Journal, are also co-founders of the firm

The Fusion GPS headquarters are registered to this office building in Northwest Washington D.C.

Fusion GPS enlisted a former British spy, Christopher Steele (pictured), to compile information for the 'dirty dossier'. The final - discredited - report cites several unnamed and allegedly highly-placed Russian sources who claim the Kremlin has been cultivating Trump as an intelligence asset for years. Steele runs the firm with fellow ex-spy Christopher Burrows (right)

Attack: How Trump responded on Friday morning to the discredited dossier

Fusion GPS is run by three former Wall Street Journal reporters: Glenn Simpson; Tom Catan; and Peter Fritsch.

Simpson was educated at George Washington University, and worked for Roll Call in Washington and the Wall Street Journal. He left there in 2009, and later spoke of his discontent at the paper after it was bought by Rupert Murdoch.

Catan grew up in the U.S. and Britain and attended the London School of Economics.

He later became editor of a business magazine in Mexico, worked for the Financial Times in New York and Argentina, and was the Spain correspondent for The Times, according to an online biography.

He covered antitrust issues while at the Wall Street Journal and also worked for it in Madrid.

Fritsch was the Wall Street Journal’s foreign bureau chief for Mexico, Brazil, Southeast Asia and Northern Europe, before becoming the paper’s national security editor.

According to the New York Times, Fusion GPS was originally hired by a Republican donor – who has not been publicly identified – to dig up dirt on Trump in 2015.

After Trump won the nomination, the firm began working with Democrats and honed in on Trump's links to Russia.

To help investigate Trump's alleged Kremlin connections, Fusion GPS enlisted a former British spy, Christopher Steele, according to the Wall Street Journal.

Steele, who had been posted in Moscow in the 1990s, currently runs a private intelligence firm - Orbis Business Intelligence - in Britain with Christopher Burrows. Both men are now in hiding.

How the two firms came to work together remains unclear. None of the three Fusion founders worked in Russia.

The 'dossier' compiled by Steele cites several unnamed and allegedly high-placed Russian sources who claim the Kremlin has been cultivating Trump as an intelligence asset for years.

According to the discredited report, the Russians recruited Trump through blackmail and by bribing him with women. It claims the Putin government has a videotape of Trump hiring prostitutes to urinate on a hotel bed in Moscow while he watched.

Although the dossier was pitched to a number of news outlets since the summer, so far no outlets have been able to verify the allegations.

The claims first went public this week, after CNN reported that US intelligence officials were looking into the dossier and had briefed Obama and Trump on its contents.

One of the allegations in the unverified and discredited report was that Trump hired prostitutes to urinate on a hotel bed in a Ritz Carlton in Moscow while he watched

At a news conference Wednesday, Trump hit back at news organizations who reported on the dossier, calling CNN 'fake news' for reporting on the questionable information

That report is disputed by NBC, which says that the intelligence chiefs did not brief Trump on it orally and gave a summary document to his Washington staff, not to him or his aides in New York.

Either way, CNN's report was the trigger for Buzzfeed to publish a copy of the report - despite acknowledging it contained errors and could not be verified.

That decision led to a firestorm of criticism for Buzzfeed and a robust denial from Trump.

He blasted the claims as 'fake news', and said he is a 'germophobe.'

Some of the assertions have also been contradicted directly. In one instance, the report claimed Trump's lawyer Michael Cohen held a 'clandestine meeting' in Prague with 'Kremlin representatives,' in August.

But Cohen said on Wednesday that he has never been to Prague.

Simpson and his company did not respond to requests for comment.

DailyMail.com can disclosed that although the report was shopped around to reporters by Democratic operatives shortly before the election, Fusion GPS had also been quietly promoting the dossier since as early as last summer, sources say.

Fusion GPS was founded by first by Simpson, shortly after he left the Wall Street Journal in 2009, and works with corporate and political clients who wanted to commission private investigative journalism.

The information these firms uncover is not necessarily intended to go public, but can be used by clients to help with lawsuits, mount pressure campaigns against competitors or negotiate better business deals.

While the vast majority of political campaigns still rely on traditional 'opposition research groups' to dig up information on opponents, that information is largely limited to public records.

Steele currently runs his own private intelligence firm in Britain, named Orbis Business Intelligence from this prestigious central London address

Spy Christopher Steele had been posted in Moscow in the 1990s and now lives in Farnham, Surrey. He has been in hiding since the story emerged.

Part of the document is seen above. Click here to see the full document first published by Buzzfeed

Campaigns have increasingly been turning to firms like Fusion GPS, which use source networks to find information that is not readily accessible or in the public domain.

While many of these firms prefer to operate in the shadows, Fusion GPS has been at the center of several high-profile political debates in the last few years.

In 2015, a group of pro-life activists released a series of undercover videos that appeared to show Planned Parenthood officials agreeing to sell fetal tissues obtained through abortions to medical researchers.

The videos set off a political firestorm, with Republican lawmakers calling for criminal investigations and for the federal government to cut off funding to Planned Parenthood.

In response, Planned Parenthood launched a defensive public relations campaign, hiring Fusion GPS to investigate the videos and the activist filmmaking group, the Center for Medical Progress.

An analysis by Fusion GPS claimed that the videos were edited in a deceptive way – a charge that was denied by the filmmakers.

Fusion GPS was also reportedly hired in 2012 to dig up information on a major Mitt Romney donor, Frank VanderSloot, such as his divorce records. The revelation led to criticism from the Wall Street Journal that Democrats were engaging in 'Nixonian' tactics by trying to smear Republican supporters.

Other clients include Denis Katsyv, a Russian businessman, who lso reportedly enlisted Fusion GPS to help defend against U.S. federal charges of money-laundering last year.

The firm's work for Katsyv led to recent allegations that Fusion GPS failed to properly register as a foreign agent, according to Politico. The firm denied this, saying it was not required to register because it did not engage in actual lobbying.