LEADING Brexiteers have begun to rally round Boris Johnson as the Tories’ internal battle over Britain’s withdrawal from the European Union looks set to reach a critical moment in New York tomorrow with a “showdown” between Theresa May and her Foreign Secretary.

Michael Gove has backed his old EU referendum campaign ally for reviving Vote Leave's controversial claim that up to £350 million a week extra would be freed up for public spending after Brexit.

The Environment Secretary, who spectacularly torpedoed Mr Johnson's run for the Tory leadership after the Brexit vote, expressed support for his Cabinet colleague and accused critics of trying to "refight" the referendum.

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The claim first attracted criticism during the referendum campaign, when Mr Johnson and Mr Gove were travelling around the country in a bus emblazoned with the slogan "We send the EU £350m a week, let's fund our NHS instead".

The Foreign Secretary's decision to revive it at the weekend in an unauthorised intervention, a 4,000-word article, prompted statistics watchdog boss Sir David Norgrove to publish a letter to Mr Johnson saying he was "surprised and disappointed", claiming it was a "clear misuse" of official figures.

Mr Johnson responded with his own letter accusing the UK Statistics Authority chairman of a "complete misrepresentation" of his views and called on him to withdraw the criticism.

But as tensions rose within Tory ranks Mr Gove went online to support the Foreign Secretary, tweeting: "In the debate on EU contributions it's important people look at what Boris actually wrote in his Telegraph article, not headlines.

"Debate should be forward looking on how to make most of life outside EU, not refighting referendum".

In his article on Saturday, Mr Johnson wrote: "Once we have settled our accounts, we will take back control of roughly £350 million a 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."

However, suggestions that Mr Johnson and Mr Gove would both resign if Mrs May conceded too much to the EU over the transition period were dismissed by a source close to the Scot.

“If there was a suicide pact of that nature, you’d think Boris would have had the courtesy to tell us he was going to jump,” he said.

But Alistair Carmichael, the Liberal Democrats’ Chief Whip, speaking at his party’s conference in Bournemouth, noted: “Boris is angling to be sacked and May is too weak to sack him.”

It is the timing of the Foreign Secretary’s intervention, which has dismayed some of his senior colleagues as it is being seen as an attempt to box Mrs May in ahead of her keynote speech in Florence on Friday when she will speak about the transition period post Brexit. Amber Rudd, the Home Secretary, berated Mr Johnson for his “back-seat driving” on Brexit.

It is thought Philip Hammond, the Chancellor, and David Davis, the Brexit Secretary, are close to agreeing a compromise, whereby Britain pays into EU coffers £10bn a year over a three-year transition period to access EU markets, paving the way to a new trading and customs deal outwith the Brussels bloc. This, it is believed, could be the meat in Mrs May’s Florence speech.

However, in his article, Mr Johnson suggested strongly that he was opposed to paying any money to the EU during a transition period, writing: “We would not expect to pay for access to their markets any more than they would expect to pay for access to ours.”

Former Cabinet minister John Redwood, a prominent Brexiteer, also came out to support Mr Johnson, arguing that there was no legal reason to continue paying money to the EU after Brexit.

He accused the Treasury of briefing that the UK would be prepared to continue paying into Brussels' coffers.

"Many of us don't think there is any moral or political or legal reason to go on paying them once we have left. Indeed, I think it would be illegal to go on paying them once we have left," he said.

The former Cabinet minister also told BBC Radio 4's Today programme it was "much better not to have a transition period".

Although he stopped short of blaming Mr Hammond personally, Mr Redwood said there had been "clear briefing out of the Treasury" that was "not in line with Government policy".

He said: "All the time there is briefing coming out of the Treasury implying they would like to pay money to the EU, the EU will dig in and offer us absolutely nothing because they will say: 'This is good, they are prepared to offer us money even to have talks with us.' That would be absolutely absurd, to do that."

Fellow Tory Brexiteer Jacob Rees-Mogg also supported Mr Johnson, making clear the UK should not agree to any "unnecessary divorce pay-out".

But Tobias Ellwood, the Defence minister, acknowledged there was now disunity within the Conservative Party over Brexit.

In a message on Twitter, Mr Ellwood, who served under Mr Johnson at the Foreign Office until June, said: "Party discord: Think many would agree we are not witnessing our finest hour, at a testing time when poise, purpose and unity are called for."

Meanwhile, a Downing Street spokeswoman said the question of Britain paying for access to the European markets was a matter for negotiations.

Asked if the UK Government was leaving the option open, she said the PM had set out in her Lancaster House speech in January that Britain would leave the single market and customs union and that there would be a time-limited implementation period, with further policy to be set out on Friday.

"As to what happens in that period or after, those are issues for the teams between the UK and Brussels to negotiate," she told a regular Westminster briefing.

The spokeswoman said Mr Johnson's views on Brexit are well known and it was important that he and the Cabinet are united behind the Government's plan for Brexit, pointing out that he specifically mentioned the Lancaster House address.

She said the PM had full confidence in Mr Johnson and would not answer questions of process on whether Number 10 has to clear ministers' articles before they are published.

Today, the row over Mr Johnson’s intervention is set to overshadow the Prime Minister’s trip to Canada for talks on trade and security with the country’s Prime Minister, Justin Trudeau. Mrs May is bound to face questions about “Bozzagate” at a joint press conference later today.

Tomorrow, she is due to have private talks with the Foreign Secretary ahead of her address on Wednesday to the United Nations General Assembly in New York.

The PM and Mr Trudeau are also expected to agree the establishment of a new joint working group, the 13th established by the UK since last year's referendum, to prepare the ground for a bilateral deal based on the EU-Canada deal soon after Brexit.

Speaking ahead of her visit, Mrs May said Canada and the UK formed a "powerful union" when they worked together on priorities like free trade.

The two leaders are also expected to discuss a trade dispute involving Canadian aerospace company Bombardier, which is a major employer in Belfast.