The former Trump staffer at the heart of allegations into Russian interference in the 2016 US presidential election has taken to Twitter to attack the former Australian foreign affairs minister and ambassador to the UK.

George Papadopoulos has accused Alexander Downer of being a "Clinton errand boy and a wannabe spy", after former US Special Counsel Robert Mueller was grilled in a congressional hearing in Washington DC on Wednesday.

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The questioning centred on a now-infamous meeting at a London bar in May 2016 between Downer, who was then Australia's high commissioner to the UK, and Papadopoulos, who was working on Donald Trump's presidential campaign at the time.

The final report into Mueller's investigation into whether anyone on the Republican Trump team conspired with Russia to discredit his Democrat opponent - and whether Trump himself then sought to obstruct justice by hindering the investigation - named the meeting in the London bar as the catalyst for an FBI investigation into alleged Russian interference that was launched two months later.

Alexander Downer on his way to meet British Chancellor of the Exchequer Philip Hammond at 11 Downing Street in London in January. Credit: Getty

Over gin and tonics at the swank Kensington Wine Rooms in London, Downer says Papadopoulos told him Russia was sitting on damaging material the could derail Hillary Clinton's campaign.

Chevron Right Icon 'It's a little bit sad.'

Papadopoulos, who was subsequently convicted of lying to the FBI, has consistently denied this allegation made by his nemesis, who he dubs "the Devil from Downunder" in his memoir Deep State Target.

RELATED: Alexander Downer was hostile and 'played me' former Trump aide says

Papadopoulos claims he was set up by Downer, who was acting for Australian, UK and US intelligence agencies, a claim the former Australian ambassador says is "a little bit sad".

George Papadopoulos arrives at the U.S. District Court for his sentencing hearing on September 7, 2018 in Washington, DC. Credit: Getty

Twitter taunt

On Tuesday, Downer appeared to be goading Papadopoulos on Twitter, by revisiting the Kensington Wine Rooms with his daughter Georgina and tagging the former Trump aide.

"With @GeorginaDowner at Kensington Wine Rooms. I’m spying on her for CIA, FBI, MI6, ASIS, ASIO and many more!!! Another fantastic conspiracy theory is born! @GeorgePapa19"

On Wednesday, Republicans peppered Mueller with questions about whether Downer had been sent to spy on Papadopoulos.

Chevron Right Icon 'Russians have dirt on Clinton.'

Ohio Republican Jim Jordan listed Downer and an Australian official, Erika Thompson, who was working with him at the time, as two of four "human sources" the FBI used to "swirl around" Papadopoulos.

"In one of these meetings, Mr Papadopoulos is talking to a foreign diplomat (Downer) and he tells the diplomat, 'Russians have dirt on Clinton'," Jordan said.

"That diplomat then contacts the FBI and the FBI opens an investigation based on that fact."

Russian connections

Papadopoulos, a 31-year-old from Chicago, pleaded guilty last year and was sentenced to 14 days' prison for lying to the FBI about his contact with Russian nationals and an inscrutable Maltese identity called Joseph Mifsud, who is believed to have high-level connections within the Russian government.

Robert Mueller, left, and Joseph Mifsud Credit: Getty

Papadopoulos claims it was Mifsud who told him Russia had dirt on Trump's presidential rival Clinton.

Chevron Right Icon 'Downer conveys a rumour he supposedly heard about a conversation.'

Devin Nunes, the top Republican on the House Intelligence Committee, suggested Downer was acting as an informant for the FBI.

"So Downer conveys a rumour he supposedly heard about a conversation between Papadopoulos and Joseph Mifsud," Nunes said to Mueller at Wednesday's hearing.

"(Former FBI director) James Comey has publicly called Mifsud a Russian agent.

"Yet your report does not refer to Mifsud as a Russian agent.

"Misfud has extensive contacts with Western governments and the FBI.

"For example, there was a recent photo of him standing next to Boris Johnson, the new prime minister of Great Britain.

"What we are trying to figure out here, Mr Mueller, is if our NATO allies or Boris Johnson have been compromised."

The hearing was shown a photo of Mifsud standing with Johnson.

The House Intelligence Committee is shown a photograph of British Prime Minister Boris Johnson (L) and Joseph Mifsud during the questioning of former Special Counsel Robert Mueller. Credit: Getty

Trump has claimed the Mueller report completely exonerated him from all allegations of Russian collusion and interference in the investigation.

In his congressional testimony on Wednesday, Mueller said he had not exonerated Trump of obstruction of justice.

But he said charges were never considered because he received legal advice that a sitting president was constitutionally immune from indictment and criminal prosecution.

However, Mueller agreed there was no legal impediment to a president being charged with a crime after leaving office.

- with AAP