Donald Trump’s latest false assertion appears to be based on nothing more than one unverified tweet spread by far-right and conspiracy websites.

Mr Trump claimed he would have won the popular vote if “millions” of illegal immigrants had not cast their ballots in the US Presidential election.

“In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally,” the President-elect tweeted on Sunday afternoon.

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But while a number of pro-Trump websites have enthusiastically reported that three million illegal immigrants voted in the US election, none provide any reliable evidence to back it up.

Instead, the majority refer to a tweet posted in the days following the election by a man called Greg Phillips, who identifies himself as the founder of an "election fraud reporting app".

Mr Phillips wrote on 11 November: “Completed analysis of database of 180 million voter registrations. Number of non-citizen votes exceeds three million. Consulting legal team."

“We have verified more than three million votes cast by non-citizens,” he added on 13 November, claiming to have analysed “180,000,000 voter registrations”.

Mr Phillips, who is also a Republican Party campaigner and founder of a technology consulting firm, has refused to share the source of his data.

When challenged to reveal the evidence behind his claims, Mr Phillips said: “No. We will release it in open form to the American people. We won’t allow the media to spin this first. Sorry.”

This information is yet to be published by Mr Phillips or his colleagues.

“In the absence of supporting data [...] the ‘three million non-citizens’ figure may just as well have been plucked out of thin air,” said fact-checking site Snopes, which investigated the claim.

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One of the websites that reported Mr Phillips's tweets as news was Infowars, a site run by radio host and conspiracy theorist Alex Jones, who has previously called 9/11 an "inside job".

It was shared more than 50,000 times on Facebook, which has faced pressure to stem the spread of unverifiable 'fake news' on the social network.

Mr Trump won the election with 290 Electoral College votes compared to Hillary Clinton’s 232.

But Ms Clinton won the popular vote, taking 48 per cent of all votes cast compared to Mr Trump’s 46.6 per cent - more than two million more than the President-elect.

There are an estimated 11.4 million unauthorised immigrants living in the United States, according to the US government.