Boris Johnson insists he is ‘all behind’ PM after sparking leadership bid claims

Boris Johnson has insisted he is “all behind Theresa for a glorious Brexit” after setting out a 4,000 word vision of Britain’s future outside the European Union that has been viewed as a challenge to her leadership.

The Foreign Secretary revived the widely-criticised claim quitting the EU could boost NHS coffers by £350 million a week and warned against paying for access to European markets in the future.

But the detailed assessment of life after March 2019 was released just six days before Mrs May sets out her Brexit blueprint in a speech in Florence, fuelling speculation about Mr Johnson’s leadership ambitions.

In a tweet linked to his article, Mr Johnson insisted, however, he was “looking forward to PM’s Florence speech”.

“All behind Theresa for a glorious Brexit,” he added.

Writing for the Daily Telegraph, Mr Johnson suggested continued membership of the single market and customs union would make a “complete mockery” of the referendum result.

He insisted Brexit would allow the UK to “be the greatest country on earth” and “our destiny will be in our own hands”.

“This country will succeed in our new national enterprise, and will succeed mightily,” he wrote.

The Leave campaign’s most eye-catching pledge during the referendum campaign was a claim that ending Britain’s contributions to the EU would free up an extra £350 million a week that could be spent on the NHS.

But it was widely derided in the weeks after the result, and Mr Johnson and other campaigners appeared to distance themselves from the promise.

In the article for the Telegraph, however, he said the UK would “roughly” be £350 million better off and it would be a “fine thing” if a lot of it went on the health service.

He wrote: “Once we have settled our accounts, we will take back control of roughly £350 million 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.”

Critics warned there is “absolutely no chance” of the £350 million pledge being delivered and said Mr Johnson was untrustworthy.

The Foreign Secretary did not mention plans for a transition period after Britain leaves the bloc and argued there is no need to pay for market access.

“We would not expect to pay for access to their markets any more than they would expect to pay for access to ours,” he said.

The Government should “seize the opportunity” of Brexit to reform the UK tax system to boost investment, he suggested.

Rules restricting foreign buyers from snapping up property could be imposed once Britain leaves the EU but the UK should not slam the door on immigration, he wrote.