BORIS Johnson has said the Brexit campaign's highly controversial claim that leaving the European Union would mean Britain gets £350 million extra a week to spend on the NHS was an underestimate.

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.

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Mr Johnson told the newspaper: "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."

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,"

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".

After reviving the argument in September, the Foreign Secretary was accused of "misusing" official figures to highlight the benefits of Brexit by UK Statistics Authority chairman Sir David Norgrove, angering Leavers.

The watchdog had already warned Vote Leave the number lacked "clarity" because it referred only to the UK's gross annual contribution and did not take into account Britain's rebate or and other payments that come back from the EU.

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Shadow Brexit secretary Sir Keir Starmer criticised Mr Johnson for again reviving his claim, claiming he has "no shame".

Mr Johnson also made clear he is not in favour of a second referendum on EU membership, although he claimed Leave would win by an even bigger margin.

It comes after former Ukip leader and leading Brexiteer Nigel Farage said he was coming round to the idea of another vote.

Mr Johnson told the newspaper: "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.

"I'm not convinced that the public is absolutely gagging for another Brexit referendum."

And he dismissed the suggestion that Brexit may not happen.

"I genuinely don't think that will happen in this case. I think that something very profound has happened in the UK.

"And I think actually were there to be - I don't think there should be a second referendum - I think the result would be pretty much the same, or the result would be more heavy for leave, I really do."

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Labour MP Alison McGovern, a leading supporter of the Open Britain campaign for close ties with the EU, said: "Our NHS is in the middle of a winter crisis and Boris Johnson's solution is to return to the scene of his previous crimes and promise ever larger slices of pie in the sky."

Eloise Todd, CEO of anti-Brexit campaign Best For Britain, said: "This is a yet another untruth 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 £350 million, that bus and that pledge for the rest of his political life."