Boris Johnson has reopened the row about the Brexit campaign's claim that leaving the European Union would give Britain an extra £350 million a week to spend on the NHS, saying the figure was an underestimate.

The Foreign Secretary said Vote Leave 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.

Johnson claimed Britain's contribution to the EU budget was already at £362 million a week, the Guardian reported.

"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 told the newspaper.

"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 attracted considerable criticism during the referendum campaign, when 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."

The campaign was accused of "misusing" official figures to highlight the benefits of Brexit.

The UK Statistics Authority, a watchdog, 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.

Johnson also said he did not favour of a second referendum, although he claimed Leave would win by an even bigger margin.

"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," Johnson said.

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

He also 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."

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."