Former Trump campaign adviser George Papadopoulos told Australian diplomat Alexander Downer in May 2016 that Russia had political dirt on Democratic presidential candidate Hillary Clinton.
The conversation between Papadopoulos and Australia's High Commissioner to the UK was a driving factor behind the FBI's decision to open a counter-intelligence investigation of Moscow's contacts with the Trump campaign, the New York Times reported on Saturday.
Two months after the meeting in London, Australian officials passed the information to their American counterparts, when leaked Democrat emails began appearing online, according to the newspaper, which cited four current and former US and foreign officials.
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Besides the information from the Australians, the probe by the Federal Bureau of Investigation was also propelled by intelligence from other friendly governments, including the British and Dutch, the Times said.
On October 30, Papadopoulos, a Chicago-based international energy lawyer, pleaded guilty to lying to FBI agents about contacts with people who claimed to have ties to top Russian officials.
It was the first criminal charge alleging links between the Trump campaign and Russia.
The White House has played down Papadopoulos' campaign role, saying it was "extremely limited" and that any actions he took would have been on his own.
But The New York Times reported that Papadopoulos helped set up a meeting between then-candidate Donald Trump and Egyptian President Abdel Fattah al-Sisi, and edited the outline of Trump's first major foreign policy speech in April 2016.
The federal investigation, which is now led by Special Counsel Robert Mueller, has hung over Trump's White House, since he took office almost a year ago.
Some Trump allies have recently accused Mueller's team of bias against the Republican president.
Lawyers for Papadopoulos and Mueller's office did not immediately respond to requests by Reuters for comment.
Trump's White House attorney, Ty Cobb, declined to comment on the New York Times report.
"Out of respect for the special counsel and his process, we are not commenting on matters such as this," he said in a statement.
Mueller has charged four Trump associates, including Papadopoulos, in his investigation.
Russia has denied interfering in the US election, and Trump has said there was no collusion between his campaign and Moscow.