Whitehall did a "c-p job" negotiating Brexit as it was biased towards the EU, says David Davis.
The former Brexit secretary said the Civil Service "sympathised with the European view" as Theresa May's government found itself in a stalemate over the terms of leaving Brussels.
Mr Davis, who resigned from Mrs May's Cabinet in July 2018 as he did not "believe" in her Chequers plan for leaving the EU, said bureaucrats should take some of the blame for the deadlock.
"Whitehall did a really c-p job of negotiation - I mean, really c-p," said Mr Davis in an interview with the Institute for Government.
"I think it's partly because they sympathised with the European view and assumed that was reciprocated. It wasn't.
"You know, if you feel the person on the other side of the table is a nice person, and you really understand their point of view, there is a tendency to think that they'll be friendly to you - which is naïve on a grand scale."
While he said Brexit would eventually "deliver", he voiced frustration with the more recent logjam over the Northern Ireland Protocol.
Mr Davis also revealed that Boris Johnson, the former prime minister, found himself "caught short" when asked to speak during crisis talks at Chequers, because he thought Mrs May would "go to somebody else first".
"Boris does everything at the last minute and he hadn't thought through the arguments, so he made a complete horlicks of it," he added.
It came as Caroline Dinenage, another former minister, said she had felt forced to steer civil servants towards "new and imaginative" thinking, as their instinct was to introduce further red tape.
"I think we've kind of got a little bit lazy - we weren't looking for a workaround, we'd just whack through a bit of legislation," said Ms Dinenage in her own reflections on her time as a frontbencher.
She added there was a "satellite delay" in the thinking of civil servants, who remained "stuck in the mindset" of having to deal with political deadlock - even once the Conservatives had a landslide majority.
Meanwhile Lord Frost, the former Brexit negotiator, said bureaucracy and so-called "woke" diversity schemes across Whitehall represented a "big burden … compared to getting the day job done".
"I know people jokingly say: 'Why is [the] Civil Service doing so much on the woke agenda?' But it really is - whether it's woke or not - a big, big burden on people who should be doing the day job."