Reality Check: Did millions vote illegally in the US?
The claim: Donald Trump would have won the popular vote in last year's US presidential election had it not been for people voting illegally.
Reality Check verdict: There is no evidence to support the assertion that at least 2.86 million people voted illegally.
White House press secretary Sean Spicer confirmed on Tuesday that President Donald Trump stands by his concerns about illegal voting.
The disclosure came after the president was reported to have claimed in a closed meeting on Monday that between three and five million unauthorised immigrants had voted for Hillary Clinton.
At the end of November, Mr Trump tweeted: "I won the popular vote if you deduct the millions of people who voted illegally."
While the president won the election via the electoral college, he actually received 2.86 million fewer votes than his rival.
So his suggestion is that at least 2% of the people who voted did so illegally, assuming that they all voted for Mrs Clinton.
Non-citizens of the United States, including permanent legal residents, do not have the right to vote in presidential elections. Voter registration requires applicants to declare their citizenship status, and they could face criminal punishment if they falsely claim citizenship rights.
In addition to being registered voters, in two-thirds of states, voters are required to bring identification to the polls in order to be allowed to vote. In all states, first-time voters who register to vote by post must provide valid identification before voting.
Donald Trump and his team have referred to two studies they say show the threat posed by unauthorised voting; both have been challenged.
A 2014 study published in Electoral Studies found evidence that suggested non-citizens do vote and "can change the outcome of close races". Donald Trump referred to this study on the campaign trail in Wisconsin on 17 October.
The research has been roundly criticised by political scientists who said it misinterpreted the data. The team behind the research used data collected by the Cooperative Congressional Election Study (CCES), which is a national survey taken before and after elections. The CCES published a newsletter that disputed the findings and said "the likely percent of non-citizen voters in recent US elections is 0".
During the campaign, Mr Trump also referred to a 2012 Pew Center on the States study that found 1.8 million dead Americans were still registered. The deceased, alleged Mr Trump, were still voting. The report, however, does not make any statements about this claim.
Although it is not impossible for non-citizens to break voting laws, there is no evidence that millions of immigrants without the right to vote influenced the outcome of the popular vote.
Election officials, including those from the Republican Party, have said there was no evidence of mass electoral fraud and senior Republicans such as House Speaker Paul Ryan have distanced themselves from the claim.
But President Trump tweeted from his personal account on Wednesday to say that he would be asking for a major investigation into voter fraud.