THE UK Government’s top civil servant in charge of Brexit has walked out after just over a year in the job.

Oliver Robbins, the Permanent Secretary at David Davis’s Department for Exiting the EU, has legged it to Downing Street to take on a “co-ordinating role”.

Opposition parties said it was proof Theresa May has “lost control of her government” after a chaotic weekend, which also saw her undermined by her Foreign Secretary, Boris Johnson.

May is to make a speech in Florence on Friday, and reports suggest some of the Brexiteers in her party are worried she may be working on a plan to keep paying cash during, and possibly after, the UK’s period of transition, for access to the single market.

On Saturday, Johnson, in what many Tories interpreted as a bid to replace May as party leader, wrote a 4000 word essay on his vision of Brexit in The Telegraph, including why he opposed paying anything during transition.

Yesterday, there were reports he would walk out of Cabinet if May was to be any softer on Brexit.

One ally told the London Evening Standard: “Brexit is his baby. He thinks to himself, ‘What’s the point if Brexit goes south? I think he feels he is in Government to get Brexit right.”

Robbins, a career civil servant, has seemingly had enough of this sort of thing. Government officials said Robbins will still take part in next week’s fourth round of Brexit talks, but he will be working for the Prime Minister rather than Davis.

A Whitehall official told the Evening Standard: “Olly has always worried the Brexiteers as to where he is. There were comments going around that he was trying to slow things down. I’ve heard it said that Olly Robbins is the man on top of all the detail, while DD does the showmanship.”

It was reported in July that the civil servant, who worked with May at the Home Office, had annoyed Davis by making decisions without him.

Reports suggest EU negotiators respected Robbins, who feel he has been “doing his best”, despite the Tory turmoil.

A Government spokesman said: “In order to strengthen cross-government co-ordination of the next phase of negotiations with the European Union, the Prime Minister has appointed Oliver Robbins as her EU Adviser in the Cabinet Office, in addition to his role as EU sherpa.

“He will continue to lead the official-side UK team in the negotiations, working closely with the Secretary of State for Exiting the European Union [Davis], and coordinate relations with the Commission and member states.”

North East Fife MP Stephen Gethin, the SNP’s Foreign Affairs spokesman said: “It’s going from bad to worse for Theresa May and her Brexit negotiations – and this resignation highlights the chaos at the heart of her government’s approach to EU negotiations. Beyond the false bravado and rhetoric, it’s clear that this is a Tory Government riddled with chaos and confusion to its very core.

“Today’s resignation is just further evidence that the Prime Minister has lost complete control of her own government.”

Johnson’s possible leadership bid was overshadowed by a disagreement over claims of exactly how much money the UK pays the EU.

The chair of the UK’s Statistics Authority, Sir David Norgrove, was forced to chastise the Foreign Secretary for claiming: “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.”

In a letter to Johnson, the independent watchdog’s chief wrote: “I am surprised and disappointed that you have chosen to repeat the figure of £350 million per week, in connection with the amount that might be available for extra public spending when we leave the European Union”.

The watchdog said the article “confused” the size of the UK’s annual gross and net contributions to the EU’s budget.

His letter continued: “It also assumes that payments currently made to the UK by the EU, including for example for the support of agriculture and scientific research, will not be paid by the UK Government when we leave. It is a clear misuse of official statistics.”

In a frosty reply, Johnson said: “I must say that I was surprised and disappointed by your letter of today, since it was based on what appeared to be a wilful distortion of the text of my article. You say that I claim that there would be £350 million that ‘might be available for extra public spending’ when we leave the EU.

“This is a complete misrepresentation of what I said and I would like you to withdraw it.”

He continued: “Once we leave the EU we will take back control of all such UK-funded spending, and, although of course I have no doubt that we will continue to spend significantly on UK priorities such as agriculture and research, that spending will be done under UK control.

“As for the rebate – whose value you did not know – it only forms part of the EU’s financing arrangements with the agreement of all other EU member states.

“We do not control it ourselves.”

The public will have to choose who to believe, the civil servant with decades of service, or Johnson, who was sacked as a journalist for making up quotes.

Labour MP Yvette Cooper said Johnson “just thinks it’s OK to repeatedly lie”.

Home Secretary Amber Rudd accused her cabinet colleague of being a Brexit “back-seat driver”.

On Twitter, Environment Secretary Michael Gove said: “Debate should be forward looking on how to make most of life outside EU – not re-fighting referendum”.