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 just 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 get ratified by member states.
In a lengthy farewell email to his staff, which has been posted on The Times website, Sir Ivan revealed that civil servants still do not know the Government's Brexit priorities and that "serious multilateral negotiating experience is in short supply in Whitehall" - unlike in Brussels.
And he 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 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.
"I hope that you will continue to be interested in the views of others, even where you disagree with them, and in understanding why others act and think in the way that they do.
"I hope that you will always provide the best advice and counsel you can to the politicians that our people have elected, and be proud of the essential role we play in the service of a great democracy."
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 in March and formal negotiations commence.
But it comes amid reports of tension between the senior diplomat and ministers, with the Daily Telegraph reporting that Theresa May and her senior team had "lost confidence" in him over his "pessimistic" view over 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 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.
"I shall advise my successor to continue to make these points."
Sir Ivan said being Britain's EU ambassador has been the highlight of his career and leaving will be a "tremendous wrench".
The email, which was also obtained by the BBC, was made public after some MPs warned that Sir Ivan's resignation showed that those who challenge Brexiteers are being increasingly frozen out.
Liberal Democrat leader Tim Farron, said: "It is damning when our own top people are slamming this Conservative Brexit Government for using ill-founded arguments and muddled thinking.
"This is the biggest decision by the UK Government in modern times and Theresa May is marching ahead without a plan or even a clue.
"We need our top people around the table if we are going to avoid wrecking the country with Brexit. It is shameful that vital, talented people like Ivan Rogers are instead being driven away."
Lord Macpherson of Earl's Court, the former Treasury permanent secretary, tweeted: "Ivan Rogers huge loss. Can't understand wilful & total destruction of EU expertise."
But former minister and Conservative MP Dominic Raab told the BBC's Radio 4 PM Programme that Sir Ivan's "heart hasn't really been in Brexit" and his resignation will be "quietly, cautiously and respectfully welcomed at the top of Government".