Britain's outgoing EU ambassador has criticised the "ill-founded arguments and muddled thinking" of politicians in his resignation letter.
Sir Ivan Rogers quit unexpectedly 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, Sir Ivan revealed civil servants still do not know the Government's Brexit priorities.
Sir Ivan added "serious multilateral negotiating experience is in short supply in Whitehall", unlike in Brussels.
He 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.
Sir Ivan ended with his thinly veiled criticism of politicians and urged his civil servants to continue to challenge ministers and "speak the truth to those in power".
The email was made public after some MPs warned Sir Ivan's resignation showed that those who challenge Brexit supporters are being increasingly frozen out.
Prominent Leave campaigner Iain Duncan Smith said he was "surprised but not shocked" at a departure that he described as "quite peculiar".
The former secretary for work and pensions was highly critical of Sir Ivan's emailed letter to colleagues, saying it suggested the civil servant was "infatuated with himself" and wanted his views on the government to go public.
"Of course he knows that (email is) going out to the media," the Conservative MP said.
Duncan Smith said there were "plenty of other really good civil servants, possibly better than him, that will get the job done for government".
Liberal Democrat leader Tim Farron said Sir Ivan's exit was "shameful" for the government.
But former minister and Conservative MP Dominic Raab told the BBC's Radio 4 PM programme 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".
In his email to staff, Sir Ivan said being Britain's EU ambassador has been the highlight of his career and leaving will be a "tremendous wrench."
His resignation comes after his second in command in Brussels, Shan Morgan, announced in November that she will be leaving the post to be the Welsh Government's Permanent Secretary.