Boris Johnson has stood by his pre-referendum promise to provide the NHS with an extra £350m a week after Britain’s exit from the European Union.

During the EU referendum Mr Johnson, who is now the Foreign Secretary, toured the country on a battlebus promising voters a £350m a week spending bonanza for the health service – alongside leading politicians in the campaign that persuaded British people to vote Leave.

But the figure has been widely disputed and the UK Statistic Authority described it as “potentially misleading” during the campaign as it failed to take into account the rebate from the EU to the UK public sector.

Mr Johnson, however, stood by his claim in a series of interviews on Thursday, claiming the £350m figure is “not disputed”.

Asked about the promise on ITV’s Good Morning Britain, he replied: “What is absolutely true is that is only if you have Theresa May conducting these negotiations – able to get the deal Britain needs – that we have any chance of taking back control of our borders, our laws and indeed huge sums of money.

Pressed on whether he still stood by the claim, the Foreign Secretary, replied: “Of course I do.” On the programme Mr Johnson was also presented on the programme with a photograph of him with the slogan: “Let’s give our NHS the £350m the EU takes every week”

He added: “The advantage of having a strong negotiator in Theresa May is that that’s the only chance we have to keep these negotiations going in a stable and ordered way with our 27 friends and partners.”

Boris Johnson defends calling Corbyn a 'mutton-headed mugwump'

Mr Johnson had previously been ambushed by a Sky News reporter brandishing a giant cheque, made out to the NHS, for £350m. But the Foreign Secretary, unsurprisingly, refused to sign it, and when asked why, he replied: “Because you’re doing a pointless stunt, that’s why.”

In a speech on Wednesday – Mr Johnson’s first intervention in the general election campaign – he warned at the London Mayor’s Banquet in the City of London that “some plaster may fall off the ceiling” in the Brexit negotiations. But, he added he was confident Theresa May “can pull it off” in his first frontline intervention in the party’s general election campaign.

Referring to the election at Mansion House, Mr Johnson added that the public needs “clear leadership to navigate this age of uncertainty” and that the Prime Minister’s decision to call the vote in June “will be a source of stability and continuity and certainty”.

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But in an article for the Sun newspaper, he stepped up his assault on the Labour leader Jeremy Corbyn and referred to him as “mutton-headed old mugwump”

In response Emily Thornberry, the Shadow Foreign Secretary, said: "It seems Boris Johnson has finally been allowed out of hiding, on the condition he only talks delusional nonsense.

"He talks about creating a 'global Britain', yet the Tories have overseen the greatest diminution of British influence on the world stage in a generation.