In a 4,000-word article for the Daily Telegraph, the Foreign Secretary also reaffirmed his commitment to the widely discredited claim that leaving the European Union will save £350m a week, which could be spent on the NHS.
The figure was emblazoned across the battle bus of the Vote Leave campaign during last year’s referendum but has been condemned as “misleading”, while other Brexiteers have distanced themselves from the contentious claim.
But the article – setting out a significant divergence from the Prime Minister’s Brexit strategy – was immediately seized upon as evidence of a forthcoming leadership bid or a prelude to a resignation. It comes just six days before Ms May sets out her Brexit blueprint in a major speech in Florence.
One senior Conservative MP told The Independent that Mr Johnson was “daring her [Ms May] to sack him” and attempting “to prove his credentials to the European Research Group in the party for an eventual leadership bid”.
Will Tanner, a former adviser to the Prime Minister, tweeted it was “astonishing” that Mr Johnson thought “this self-serving posturing, disloyal at best of times, would reflect well in the hours after a terror attack”.
“The real PM just raised the threat level,” he said. “Meanwhile guy who wants to replace her issues a prelude to resignation, to save face over £350m. Hmm.”
Mr Johnson also faced considerable criticism for the outburst on the same day of a terrorist incident on the London Underground. Accompanied by an image of the Foreign Secretary leaving Downing Street, the article appeared just hours after the Prime Minister raised the country’s threat level to critical – the highest possible level, suggesting another attack might be imminent.
Shortly after Mr Johnson’s article was published Ruth Davidson, the Scottish Conservative leader, appeared to be referring to the Foreign Secretary when she posted on Twitter: “On the day of a terror attack where Britons were maimed, just hours after the threat level is raised, our only thoughts should be on service.”
Jeremy Corbyn’s spokesperson said the article “laid bare the conflicts at the heart of Theresa May’s Government” over the Brexit negotiations and “cut the ground from beneath the Prime Minister’s authority”.
They added: “In the process he has exposed the Tories’ real Brexit agenda – a race to the bottom in regulation and corporate tax cuts to benefit the wealthy few at the expense of the rights of the rest of us.
“The Foreign Secretary even has the gall to dredge up the fantasy of £350m a week extra for the NHS. The Prime Minister must spell out now how this will be paid for, or stand condemned for once again trying to mislead the British public.”
In the article Mr Johnson wrote: “Once we have settled our accounts, we will take back control of roughly £350m per week. It would be a fine thing as many of us have pointed out if a lot of that money went on the NHS, provided we use that cash injection to modernise and make the most of new technology.
“One of the advantages of investing in the NHS – if we combine that investment with reform – is that we can turbo charge the role of our health service in driving bioscience.”
Labour’s Chuka Umunna, leading supporter of Open Britain, which is campaigning for continued single market membership, added: “Boris Johnson had a chance to vote to deliver the £350m extra a week for the NHS in February, and he refused to do so.
“He promised to guarantee the rights of EU citizens living in Britain, and nothing has happened. No one can trust a word he says.
“The £350m a week promise was a fib, and there is absolutely no chance of it being delivered. Boris should be apologising for his disgraceful conduct in the referendum, not continually making the same impossible promises.
“He’s like an old rocker who sings the same tunes, but they just don’t sound right anymore. A period of silence on his part would be welcome.”
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