Boris Johnson's comment that the Leave campaign under-estimated when it claimed Brexit would allow an extra £350 million to be spent on the NHS weekly has been branded "ridiculous".
Leading health expert Professor Sir Michael Marmot said the Brexit campaign's highly controversial claim has already been confirmed "a lie".
The academic also said the history books would remember Mr Johnson as "one of the best guests on Have I Got News For You".
The Foreign Secretary claimed the official Vote Leave campaign could have used a higher figure as the UK's gross contribution would rise to £438 million by 2021, the last year of an expected transition period.
According to the Guardian, he claimed Britain's contribution to the EU budget was already at £362 million a week.
Mr Johnson told the newspaper: "There was an error on the side of the bus. We grossly under-estimated the sum over which we would be able to take back control."
He added: "As and when the cash becomes available - and it won't until we leave - the NHS should be at the very top of the list."
Speaking at a press conference in London about reducing health inequalities, Sir Michael was asked what he made of Mr Johnson's claim.
"I think, when the history of the 21st century is written, Boris Johnson will go down as one of the best guests on Have I Got News For You.
"It's a ridiculous thing to say. It's been pointed out by the statistics authority, the national statistics authority that that is totally misleading," he said.
Sir Michael added: "He does say 'gross'. Gross is a good word to use actually ... but that's not the relevant bit."
The academic said the claim has been shown to be "a lie", adding: "It's an untruth. It's misleading. I'm not sure what other word to use."
The claim first attracted criticism during the referendum campaign, when Mr Johnson was travelling around the country in a Vote Leave bus emblazoned with the slogan "We send the EU £350 million a week - let's fund our NHS instead".
Also speaking at the press conference, David Taylor, emeritus professor of pharmaceutical and public health policy at UCL, said people who voted Brexit want a well-funded health service and it would be "tragic" if they were being misled.
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