Theresa May is expected to appoint an EU ambassador who “believes in Brexit” in the wake of the current Brussels representative's decision to quit after being cut adrift by Downing Street.
Sir Ivan Rogers on Tuesday announced his resignation as Britain’s ambassador in Brussels after it was made clear Mrs May and her senior team had “lost confidence” in him over his “pessimistic” view of Brexit.
Government sources made clear that Sir Ivan had “jumped before he was pushed” and that Number 10 believed his negative view of Brexit meant that he could not lead the negotiations after the Prime Minister triggers Article 50.
He called on his staff to challenge "ill-founded arguments" and said that "serious multilateral negotiating experience is in short supply in Whitehall".
The implicit criticism of Mrs May will infuriate Downing Street, which had on Tuesday insisted it would not comment on Sir Ivan's resignation.
Sir Simon Fraser, the former head of the Diplomatic Service who worked with Sir Ivan for "many years", warned that Britain was losing one of its biggest experts on Europe months before "very complex" Brexit negotiations begin.
He told BBC Radio 4's Today programme: "He is a highly intelligent, knowledgeable and experienced official and one of the greatest experts, if I can use the expert word, that we have on European matters in the British Civil Service."
He went on: "I do think that his sort of in-depth knowledge and expertise is a loss as we go into what is going to be, as (Brexit Secretary) David Davis himself has said, a very complex set of negotiations."
Sir Simon, who left his post in July 2015, rejected suggestions that Sir Ivan was not tough enough in negotiations, including David Cameron's attempt to reshape Britain's relationship with the EU before the referendum.
The ex-diplomat insisted Sir Ivan "called a spade a spade" in his advice to ministers.
He said: "Anyone who knows Ivan, who's worked with him, will know absolutely that he was not someone who was ready to take no for an answer.
"He was a very persistent negotiator, he showed lots of determination and he worked incredibly hard to achieve the Government's objectives."
It is understood that senior Government ministers including Boris Johnson, the Foreign Secretary, now believe that Sir Ivan’s replacement must be someone who backs the Brexit cause “wholeheartedly”.
A Government source said that the Prime Minister wants the next EU ambassador to be someone with the “same attitude [to Brexit] as the current Government”.
Senior Conservatives said that Sir Ivan’s resignation, which Number 10 was not warned about in advance, must now be seen as an opportunity to appoint a “Brexiteer” who will fight to get a good deal from Brussels.
In his email, Sir Ivan wrote to his staff: "I hope you will continue to challenge ill-founded arguments and muddled thinking and that you will never be afraid to speak the truth to those in power. I hope that you will support each other in those difficult moments where you have to deliver messages that are disagreeable to those who need to hear them."
He added: "We do not yet know what the Government will set as negotiating objectives for the UK’s relationship with the EU after exit. There is much we will not know until later this year about the political shape of the EU itself, and who the political protagonists in any negotiation with the UK will be."
Sir Ivan prompted anger from Eurosceptics in December after it emerged he had told ministers that Brexit could take a decade and may not ultimately succeed.
The leak of the private memo led to Sir Ivan, who was appointed by David Cameron in 2013, being described by Tory MPs as a “gloomy pessimist” and there were calls for his resignation.
Sir Ivan is said to have been “stung” by Downing Street’s failure to back him during the dispute and express full confidence in his ability.
"As a civil servant, when the Number 10 press machine won't stick up for you, then that's very difficult to live with," a Foreign Office colleague of Sir Ivan's said.
He also had a series of “somewhat frosty” encounters with Mrs May and her senior team at a Brussels European Council meeting in December, which is said to have convinced him he should resign.
One Whitehall source also claimed that David Davis, the Brexit Secretary, “wasn’t Ivan’s biggest fan” and that there had been tensions between the pair.
Sir Ivan, a central member of Mr Cameron’s EU negotiating team which failed to keep Britain in the bloc, had been expected to leave his post in October but is understood to have come to the conclusion that his position had become untenable.
Government sources last night made clear that his decision to quit “will not alter the Brexit timetable in any way”.
Conservative MPs welcomed his decision to stand down. However, Remain supporters said his departure is “not good for the country”.
Iain Duncan Smith, the former work and pensions secretary, said: “It's not a surprise he’s going, he clearly didn't agree with leaving the EU.
“I think its vitally important this is an opportunity not a problem, an opportunity for the UK to appoint somebody who is determined to drive through Theresa May's agenda. So it's a real opportunity to get the right person into the job who actually believes in the UK outside of the EU and is strong enough to be able to get the right kind of deal for the UK as we go forward.”
Dominic Raab, a former justice minister and senior Leave campaigner, added: "Sir Ivan is a distinguished diplomat with a long record of public service. He didn't exactly hide the fact that his heart wasn't in Brexit, and he was due to step down in the autumn anyway. It makes sense all round to give the ambassador who will see the negotiations through some lead time. And we need someone who is really up for this, with a 'can do' attitude."
And Steve Baker, the Tory MP for Wycombe, added: “The position should be filled by someone with genuine enthusiasm for our EU exit in addition to EU expertise.”
Lord Macpherson of Earl’s Court, who was until last year the permanent secretary to the Treasury, criticised Number 10 for its role in Sir Ivan’s departure.
He wrote on Twitter: “Ivan Rogers huge loss. Can’t understand wilful & total destruction of EU expertise, with Cunliffe, Ellam & Scholar also out of loop. #amateurism”.
Lord Macpherson was referring to Jon Cunliffe, deputy governor of the Bank of England, Michael Ellam, a former Treasury official now at HSBC, and Tom Scholar, successor at the Treasury who is understood to have no direct role in the upcoming Brexit negotiations.
George Osborne, the former chancellor who backed the Remain campaign, called Sir Ivan a “perceptive, pragmatic and patriotic public servant”.
Nick Clegg, the former deputy prime minister, said: “The trend seems to be for people who haven't drunk the Brexit Kool-Aid, they are increasingly being pushed to the margins.
"And that's not good for the country, it's not good for a workable negotiation on Brexit, and nor is it actually good for the informal checks and balances that exist in a mature democracy such as ours."
And Peter Mandelson, the former Labour cabinet member and senior Remain campaigner, added: “In terms of knowledge and experience of the EU, Sir Ivan Rogers is second to none in Whitehall. His resignation is a serious loss for us in Brussels.
“I would not expect him to comment further but everyone knows that civil servants are being increasingly inhibited in offering objective opinion and advice to Ministers.”
A post-Brexit trade deal between Britain and the European Union might take 10 years to finalise and could still fail, the United Kingdom's ambassador to the bloc has told Prime Minister Theresa May's government, the BBC reported on Thursday.