The former British spy behind a controversial dossier about Donald Trump's links with Russia has spent two days this week talking to a team of US investigators.

Christopher Steele met colleagues of Robert Mueller, the special counsel investigating alleged Russian election meddling, according to the Washington Post.

It means that investigators will be able to judge at first hand whether they think the claims reported by Steele in the dossier are trustworthy.

Steele had previously refused to appear before Congressional committees looking into how Russia may have interfered in the 2016 election.

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The Post also reported that Steele had compared possessing the information he found about Trump to "sitting on a nuclear weapon".

The newspaper's 4000-word article provides the fullest picture yet of how Steele acted after uncovering claims that the Russians had compromising material on Trump.

Steele and his "dossier", a series of memos written after he was given funding by first Republican and then Democrat opponents of Trump, lies at the heart of the row over Russian interference in the race for the White House. Among the claims made was that Trump, while in Russia, asked prostitutes to conduct lurid sex acts. Trump has denied the allegations.

Steele spent two decades working for Britain's Secret Intelligence Service, or MI6, including a stint in his mid-20s in Moscow, where he served undercover in the British Embassy.

When he returned to work for the agency in London, he provided briefing materials on Russia for senior government officials and led the British inquiry into the mysterious 2006 death in London of Alexander Litvinenko, a former KGB official and Putin critic. He left MI6 in 2009.

Steele worked on the dossier on behalf of Fusion GPS, a private Washington research firm.

More than a year after the dossier's completion, it remains unclear whether authorities have corroborated Steele's specific allegations about Trump's connections to Russia.

However, the US intelligence community has concluded that the Russians engaged in an elaborate operation to swing the election to Trump.

The dossier, published by Buzzfeed after the election, has become the focus of a partisan battle over the Russian investigation, which is looking into links with the Trump campaign team.

Republicans have sought to portray Steele as politically motivated and his claims as unfounded, indicating the entire Russian investigation is constructed on his faulty intelligence.

Democrats have painted Steele as someone who passed on concerns in good faith and stressed his information was not the only reason for starting the Russia investigation. The Post described how Steele, a Russian expert so trusted that he had provided briefings for UK prime ministers and at least one other US president, got drawn into the Trump case.

It went on to describe how after Steele's consulting firm, Orbis Business Intelligence, was commissioned to look into Trump, he became increasingly concerned by the discoveries coming from his network of informants.

Steele eventually reached out to the FBI, with whom he had worked to expose corruption at world football's governing body, Fifa.

He met Post journalists twice before the election to get them to print the claims, once "visibly agitated".

The Post, however, declined to publish as they were unable to verify his claims.

Around the same time, Steele also met with other news organisations including the New York Times, the New Yorker and Yahoo News, according to court filings.

FBI officials did not know Steele had spoken to Yahoo, according to a declassified version of a criminal referral released on Wednesday by two Republican senators, which they suggested meant Steele had lied about his media contacts.

During his investigation, Steele contacted old intelligence colleagues for advice, apparently convinced the information he had found was significant to national security. Richard Dearlove, former head of MI6, met Steele and his colleague at London's Garrick Club.

Sir Andrew Wood, a British former diplomat and friend of Steele, was also asked for his thoughts. "He wanted to share the burden a bit," Wood told the Post.

Republican Senator Lindsey Graham, a longtime member of the Senate Judiciary Committee, said of Steele's dual obligations - to his private clients, who were paying him to help Clinton win, and to a sense of public duty born of his previous life: "You can be an FBI informant. You can be a political operative. But you can't be both, particularly at the same time."

Around the time Steele spoke to media outlets he also spoke with then-Associate Deputy Attorney General Bruce Ohr. House Republicans claimed in a memo released last week that the British former spy told Ohr that he "was desperate that Donald Trump not get elected and was passionate about him not being president".

The GOP memo argued that Steele's comments to Ohr were "clear evidence of Steele's bias", saying they should have been noted in a warrant application that the Justice Department submitted that included his research.

Friends of Steele said his comment was not driven by political bias, but by his alarm after sifting through months of reports about Trump's ties to Russia.

After Trump won, an ally of John McCain, the Republican senator, visited Britain to meet Steele and read the dossier for himself. He was reportedly told to "look for a man wearing a blue raincoat and carrying a Financial Times under his arm" at Heathrow Airport.

The dossier was eventually passed to McCain.

Christopher Steele

Born: Saudi Arabia, 24 June 1964 (age 53)
Occupation: Researcher, former MI6 employee
Education: Social and political sciences at Girton College, Cambridge University
Career: Recruited by MI6 immediately on graduation. His 20-year career included undercover postings to Moscow, Paris and briefing trips to Afghanistan. By 2006 he was head of MI6's Russia desk and investigated the Alexander Litvinenko poisoning.
• In 2009 he founded a private investigation agency, specialising in Russian and Ukrainian issues. Among his work was the investigation of corruption at Fifa and of its head Sepp Blatter.
• In 2015 he was employed by the Democratic National Committee to conduct research on presidential candidate Donald Trump. The resulting dossier detailed the Trump campaign's alleged collusion with Russia and its incendiary contents became an open secret in media circles.
• On publication of the dossier, Steele briefly went into hiding.
• He is currently facing investigation from the US Department of Justice, at the instigation of Senate Judiciary Committee chairman Chuck Grassley.