BRITAIN'S outgoing EU ambassador has hit out at the "ill-founded arguments and muddled thinking" of politicians in his shock resignation letter.

Sir Ivan Rogers unexpectedly quit months after he sparked controversy by warning the Government that a post-Brexit trade deal could take a decade to finalise, and even then may fail to be ratified by member states.

In a lengthy farewell email to his staff, obtained by The Times and the BBC, Sir Ivan said civil servants still do not know the Government's Brexit priorities and "serious multilateral negotiating experience is in short supply in Whitehall" - unlike in Brussels.

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."

In his resignation letter, Sir Ivan criticised politicians and urged his civil servants to continue to challenge ministers and "speak the truth to those in power".

Sir Ivan wrote: "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."

In the email, sent just before 1pm on Tuesday, Sir Ivan said he decided to step down early so his replacement can be in place when Article 50 is triggered by April and formal negotiations begin.

But it comes amid reports of tension between the senior diplomat and ministers. The Daily Telegraph reported that Theresa May and her senior team had "lost confidence" in him over his "pessimistic" view of Brexit.

Sir Ivan stressed the need for expert civil servants to play a central role in the negotiations and urged his staff to tell ministers the true opinions of the other 27 member states "even where this is uncomfortable".

He wrote that "we do not yet know what the Government will set as negotiating objectives for the UK's relationship with the EU after exit" but the UK's permanent representation to the EU (UKREP) must be "centrally involved in the negotiations if the UK is to achieve the best possible outcomes".

He added: "Serious multilateral negotiating experience is in short supply in Whitehall, and that is not the case in the (European) Commission or in the Council.

"The Government will only achieve the best for the country if it harnesses the best experience we have - a large proportion of which is concentrated in UKREP - and negotiates resolutely.

"Senior ministers, who will decide on our positions, issue by issue, also need from you detailed, unvarnished - even where this is uncomfortable - and nuanced understanding of the views, interests and incentives of the other 27."

Sir Ivan also said the allocation of roles in the UK's negotiating team needs "rapid resolution" and hit out at assertions by some politicians that a free trade deal will be easy to negotiate.

He said: "Contrary to the beliefs of some, free trade does not just happen when it is not thwarted by authorities: increasing market access to other markets and consumer choice in our own, depends on the deals, multilateral, plurilateral and bilateral that we strike, and the terms that we agree."

The email was made public after some MPs warned that Sir Ivan's resignation showed that those who challenge Brexiteers are being increasingly frozen out.

Shadow international trade secretary Barry Gardiner called Sir Ivan an "extremely distinguished civil servant who knew the EU very well".

Mr Gardiner told BBC Breakfast: "When he says, 'look, this is what these people are thinking, this is what these people are going to try and do', you take that as intelligence that you then need to factor into your own negotiating position.

"It really is, I think, dangerous for the Government to rubbish people like that."

Liberal Democrat leader Tim Farron said it was "damning when our own top people are slamming this Conservative Brexit Government for using ill-founded arguments and muddled thinking".

Jonathan Powell, Tony Blair's former chief of staff, warned the Prime Minister against hiring a pro-Brexit successor to Sir Ivan.

Appointing a "patsy" who does not explain plainly what the other side is thinking will doom the Brexit negotiations to failure as ministers will be operating in a "fantasy land" about what is achievable, he said.

It would also break the rule that civil servants must be politically neutral and lead to the "collapse" of the entire independent system.

Mr Powell told the Today programme: "If you are not prepared to have the argument, if you are not prepared to have someone who will tell you what the problems are, you are going to end up in a disaster and I'm afraid that's what's going to happen with these negotiations if you really go for a patsy."

He went on: "The Prime Minister, the ministers, if they do not have civil servants telling them honestly what the other Europeans think, not telling them honestly what is possible to negotiate, they will live in this fantasy land of what's possible, they will live in a Daily Mail world of what could be achieved and they will fail."

But prominent pro-Brexit MP Iain Duncan Smith suggested Sir Ivan's views were less relevant as EU member states will inevitably be feeding him their most hardline views before negotiations begin.

The former Cabinet minister said civil servants are now having to "tear up the rulebook" for how they normally operate to deal with Brexit.

He told the programme: "They are now having to accept and understand that we are leaving and that means therefore sometimes the views and the opinions of what you keep feeding back from various member states isn't actually sometimes quite relevant."

He went on: "It's well and good, absolutely right, to feed that back, but ministers have to sift that and decide ultimately no, what we are going to do is this, and we therefore have to get on and do it like this.

"If you don't agree, and I have full respect for him, then you have to go."

Mr Duncan Smith also suggested Sir Ivan had undermined his position by "going public" too often.

"This is now the second time - it may actually prove that ministers may well be right to say that they weren't prepared perhaps to trust him quite the way they would have done with others," he said.