THERESA May has slapped down Boris Johnson, insisting that the UK Government is “driven from the front” and not by the Foreign Secretary acting as a back-seat driver.

But as the Prime Minister sought to reassert her authority, preparing for a showdown with her Cabinet colleague at the United Nations in New York today, Mr Johnson appeared unrepentant.

He issued a fresh warning about how there could be no extended transition period after Britain left the EU and declared: "We certainly don't want to be paying in extortionate sums for access to the single market. They wouldn't pay for access to our market."

Sidestepping questions on whether or not he would resign if he did not get his way, the Secretary of State said: "You are barking slightly up the wrong tree here."

He added: “As far as back-seat driving…honestly, there’s one driver in the car. It’s Theresa.”

Earlier Mrs May, en route to Canada for talks on trade and security, was asked if she was frustrated with her Mr Johnson. She replied: “Boris is Boris. I'm clear that what the Government is doing and what the Cabinet is doing is that we...base our negotiations on the principles set out at Lancaster House."

She went on: "This Government is driven from the front and we are all going to the same destination because we are all agreed on the basis of the Lancaster House speech.

"We are all agreed as a Government about the importance of ensuring the right deal for Britain, the right withdrawal agreement but also the right deal on a special partnership between the EU and UK for the future.

"We are all optimistic about what we can be achieving for the UK for the future," added the PM.

In a separate development, Mrs May sought to take more personal control of the Brexit process with the appointment of a senior Whitehall official leading Britain's negotiating team in Brussels as her adviser on the European Union.

Oliver Robbins has been moved from Brexit Secretary David Davis's department to the advisory position in the Cabinet Office but will "continue to lead the official-side UK team" in negotiations, a UK Government spokesman said.

At the UN, where the PM will speak on Wednesday, Mr Johnson, who was warmly welcomed at the UN by US President Donald Trump, sought to play down reports he was at odds with Chancellor Philip Hammond and Mr Davis.

He described his controversial 4,000-word unauthorised article as an "opening drum roll" for the PM’s keynote speech in Florence on Friday.

"Let's not try and find rows when there really aren't rows," he declared. "I was involved in that Brexit campaign quite a lot. People want to know where we are going. It is a good thing to have a bit of an opening drum roll about what this country can do."

Meanwhile, Mrs May stopped short of claiming unanimous backing for her forthcoming speech, in which she is widely expected to offer a compromise on Britain's financial settlement with the EU.

Following suggestions that she will say Britain is ready to pay as much as £10 billion a year to the EU during a three-year transition period, Mr Johnson made clear in his Daily Telegraph article at the weekend that he believed no payments whatsoever should be made after Brexit.

The PM conspicuously did not rally to the Foreign Secretary’s defence in his row with Sir David Norgrove, the Chairman of the UK Statistics Authority, who has accused him of a "clear misuse" of official figures in his claim that Brexit will give the UK control of £350 million a week, much of which could be spent on the NHS.

Asked if she trusted Sir David or Mr Johnson to be right on the figures, Mrs May said only: "The reality is that year on year, the money the UK pays into the European Union changes because of a whole variety of factors."

She made clear that the decision on whether or not any money freed up by Brexit should go into the NHS had not yet been made and was not one for Mr Johnson alone to decide.

Earlier, the Foreign Secretary received the backing of his Cabinet colleague Michael Gove, the Environment Secretary, who worked closely with him on the Vote Leave campaign but dramatically fell out with him over the Tory leadership battle.

Amid the row over the claim of a £350m a week Brexit dividend, the Scot tweeted it was "important people look at what Boris actually wrote" in his article and the "debate should be forward looking on how to make most of life outside EU; not refighting referendum".

But Tobias Ellwood, the Defence minister, publicly acknowledged there was "discord" in Conservative ranks over Brexit, tweeting: "Think many would agree we are not witnessing our finest hour, at a testing time when poise, purpose and unity are called for."