More details are emerging about the 23 January meeting between Donald Trump and the leaders of the House and Senate, where the President was said to repeat false claims that millions of people voted illegally in the 2016 election.

If true, Trump's claim would amount to the largest electoral fraud in American history. When one of the Democrats questioned what proof Trump had of this massive voter fraud, he defended his claim by telling an anecdotal story that German golfer Bernhard Langer, who Trump described as a friend, had told him.

Trump said the very famous golfer, Bernhard Langer – a German citizen who is ineligible to vote – told him recently of how he was standing in line at a polling station on Election Day, near Boca Raton, Florida, when he was turned away from the voting booth by an official.

Trump said that Langer witnessed people in line that didn't look like they were legally allowed to be at the polls as evidence to support his voter fraud claim.

The story and details were told to The New York Times by three staff members who were in the room during the meeting between Trump and the lawmakers.

But a senior White House staff member, who had also heard Trump tell the story on other occasions, told The Times that the story was actually told to Langer by one of his other friends. Langer told Trump the story in Florida during the Thanksgiving break in November 2016, the White House official said.

During an interview with ABC News on Thursday Trump repeated his claim that 3 to 5 million illegal votes were cast in the American election. "I said it. And I said it strongly because what's going on with voter fraud is horrible," Trump said, adding that his meeting with Congressional leaders was "confidential" and that the Democrats weren't "supposed to go out and talk to the press" about what happened inside.

Trump again quoted a 2012 Pew study, as he did on the campaign trail, that found "more than 1.8 million deceased individuals are listed as voters." The report identifies that America's voter registration system needs an upgrade, but it does not go as far as to say that the system is compromised and open to abuse.

"Lots of people are saying they saw things happen. I heard stories also," Trump told ABC News reporter David Muir. "You take a look at the registrations, how many dead people are there?" Trump said. "I didn't say there are millions. But I think there could very well be millions of people."

Trump wrote on Twitter before the interview aired on 25 January that he "will be asking for a major investigation into voter fraud." He singled out those registered to vote in two states, those who are illegal, and dead registered voters as holding the key to his claim. "Depending on results, we will strengthen up voting procedures!" he said.

However "millions of illegal votes would require a conspiracy larger than all previous conspiracy theories combined," said non-profit investigative journalism outfit Pro Publica when Trump made similar claims in November 2016. Pro Publica had a team of 1,100 people monitoring the vote on election day and "saw no evidence the election was 'rigged'".