Sir Ivan Rogers, the former UK ambassador to the EU, held secret talks with ex-prime minister David Cameron before Christmas over Brexit.
The meeting has raised concerns that senior government officials were privately in touch with leading Remain supporters, The Times reports.
In the meeting held towards the end of 2016, Sir Ivan is said to have told Cameron that May was not doing enough to prepare for the risk of the UK making a "disorderly" departure from the EU.
Sir Ivan resigned last week, just months before May is expected to trigger Article 50 to start Brexit talks.
The Times quoted a source familiar with what was discussed between the two as saying: "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," the source added.
According to the newspaper Rogers had told friends that he believes a hard Brexit will lead to "mutually assured destruction" between Britain and the rest of the EU.
May, in an interview with Sky News on Sunday (8 January), said that the government's thinking on Brexit "isn't muddled at all". She said that while Sir Ivan had been a dedicated civil servant, Brexit was about "more than one individual".
Sir Ivan resigned from the civil service on 7 January after he quit as EU ambassador after it was made clear that the ministers did not want to employ him in another position, the newspaper said,
May allegedly freezing out key ministers on Brexit
The Times said that there have been claims that May was not keeping key ministers in the loop over Brexit with insiders saying that she had commissioned briefing papers from senior officials which even her own three Brexit ministers have never seen.
May is said to be given private advice by Oliver Robbins, a senior official at the Brexit department. A senior Tory told the newspaper: "Brexit material goes direct to Theresa May and No 10, and [David] Davis doesn't see it. It's quite possible he has no idea."
May will be holding her first cabinet committee meeting this year on the EU on Thursday 12 January. At the meeting, ministers are expected to finalise their strategy should the government lose the Supreme Court case that will force it to pass legislation before it can trigger Article 50.