Boris Johnson defends the Leave Campaign's contentious claim that Brexit will create an extra £350 million a week to spend on the NHS.
"We grossly underestimated the sum," Johnson told The Guardian.
Independent fact-checkers have debunked the 2016 claim as misleading.
Labour accuses the foreign secretary of having no shame.
LONDON — Boris Johnson has defended the Leave campaign's claim that Brexit will give Britain an extra £350 million a week to spend on the NHS by arguing the campaign could have used a larger figure.
The foreign secretary has told The Guardian that the figure splashed across the side of the Leave campaign's battle bus in the run-up to the 2016 Brexit referendum should have been bigger than £350 million.
Johnson was pictured standing in front of the bus which had the slogan "we send the EU £350 million a week, let's fund our NHS instead" emblazoned on the side.
In 2016 the UK Statistics Authority described the £350 million claim as "potentially misleading" as it didn't take into account the rebate which is applied before Britain pays its contributions to the European Union.
FullFact described the claim as a "clear misuse of official statistics" in September 2017.
However, Johnson has doubled-down on the contentious claim.
"There was an error on the side of the bus. We grossly underestimated the sum over which we would be able to take back control," Johnson, who played a lead role in the Brexit campaign, told The Guardian.
He claimed that the UK's gross weekly contribution to the EU will rise to £438 million by the end of the proposed two-year transition period in 2021.
The government will be able to "take back control" of this money and spend it on the NHS, 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."
Labour shadow Brexit minister, Matthew Pennycook, accused Johnson of having no shame.
"He spent the entire referendum campaign standing in front of his red bus with a bogus claim on the side, and now he is saying the figure should be higher. The public really do deserve better from the foreign secretary," he said.
Eloise Todd, CEO of anti-hard Brexit group Best For Britain, said: "This is a yet another un-truth from Boris, a man who has become so obsessed with the lie he slapped on the side of the bus.
"You have the sense that Boris will be arguing about £350m, that bus and that pledge for the rest of his political life.
"He sold Brexit on a false prospectus and with the NHS in crisis people are rightly asking where is the money and if it's not forthcoming they should have the right to change their mind. The man is a snake oil salesman."
In the same interview, Johnson played down the chances of a second referendum on Britain's EU membership.
"We’ve just had one, and I think it went pretty well but it was something that caused an awful lot of heartache and soul-searching, and everybody went through the wringer on it," he said.
"I’m not convinced that the public is absolutely gagging for another Brexit referendum."