Sir Ivan Rogers reportedly criticised Theresa May's approach to Brexit during talks with her predecessor weeks before quitting as the UK's ambassador to the European Union.
The diplomat, who has resigned from the civil service with immediate effect after leaving the post in Brussels, held talks with David Cameron before Christmas during which he voiced concerns that the Prime Minister risked heading for a "disorderly" exit from the EU.Sir Ivan has told friends that he fears a so-called hard Brexit would lead to "mutually assured destruction" for the UK and EU, The Sunday Times reported.
The newspaper said the pair dined together towards the end of 2016 and quoted a "source familiar with the conversation" as claiming that Sir Ivan fears the UK is heading for a "car crash".
The source said: "Rogers spoke to Cameron. His biggest fear was that the biggest issue is not hard or soft Brexit, but whether we have an orderly or a disorderly Brexit.
"He thinks we are heading for a car crash, where we don't get a deal and we crash out with nothing.
"Downing Street's view was that he should stop being such a pessimist.
"Rogers thinks we need to plan for a disorderly Brexit on our terms rather than theirs. No 10 has not given that the priority it deserves."
Sir Ivan shocked Westminster and Brussels with his resignation as permanent representative on Tuesday and used an email to staff to criticise "muddled thinking" over Brexit.
The Foreign Office confirmed he had now left the civil service entirely.
A spokeswoman said: "He did not seek any further civil service appointment and has therefore resigned from the civil service with immediate effect.
"We are grateful for Sir Ivan's work in Brussels and across a number of other senior positions in the civil service."
It is understood that Sir Ivan will receive three months' pay in lieu of notice, in line with standard Foreign Office terms, but no special "pay off" was offered or sought.
He is being succeeded by career diplomat Sir Tim Barrow, a former ambassador to Russia described by No 10 as a "seasoned and tough negotiator" who will help the Government make a success of Brexit.
In a fiery message to staff announcing his resignation from the Brussels post, Sir Ivan had hit out at the "ill-founded arguments and muddled thinking" of politicians and said civil servants still did not know the Government's plans for Brexit.