Theresa May has shown “culpable naivety” over the Brexit talks, while leavers such as Boris Johnson and Jacob Rees-Mogg are “still lost in campaign mode on fantasy island”, the UK’s former ambassador to the EU has said.

In his most outspoken public comments yet on Brexit, Sir Ivan Rogers gave a withering account of how the prime minister has handled the process, while giving an even more scathing dismissal of the hopes of Brexiters for a “super Canada” deal.

“Nearly two and a half years on from the referendum, we are, in other words, both on the EU deal, and on other post-Brexit trade deals, still lost in campaign mode on fantasy island,” he told an audience at Cambridge University on Wednesday night.

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While not naming May, he described her policy as unwise and said there had been an “extraordinary … [and] culpable naivety” in the attempt to get “all the benefits of unchanged free trade from its EU membership days, with none [or] few of the obligations”.

In the speech entitled “Brexit as a revolution”, Rogers suggested the prime minister’s attempt to placate hardliners came at the price of red lines that boxed her in before the Brexit negotiations had begun.

“As a quiet ex-remainer, she might be viewed by her party’s ‘true Brexit’ wing as a dangerous counterrevolutionary whose heart was not truly in the Brexit project,” he said of the “citizens of nowhere” speech given by May at the 2016 Conservative party conference.

He recalled surprise among “European elites that the PM should have taken such a hardline and unequivocal view on the UK’s post-Brexit destination”.

“The EU reacted, as was completely inevitable, by averring that her red lines, if immutable, pointed ineluctably to an economic relationship no deeper than a bog-standard free-trade area,” Rogers said.

“It was never going to say anything else. It was never going to change its legal order for the benefit of a state that had chosen to leave it.”

While he said there was “zero chance” of the EU agreeing to the economic elements of the prime minister’s Chequers plan, Rogers was equally blunt about ideas promoted by Johnson and the “pinstriped Robespierres” of the European Research Group, led by Rees-Mogg. “The Johnsonian Canada-plus-plus is as big a pipe dream as Chequers. In some respects, rather bigger,” he said.

Rogers resigned as ambassador in January 2017 after being dismissed as “the gloomy mandarin” by Tory Eurosceptics, who disliked his warnings that leaving the EU would be an extremely complicated process that would dominate UK political life for a decade.

Last month, Rogers warned Brexit negotiators on both sides risked “sleepwalking into a major crisis”, but his latest speech is his most outspoken critique yet of the people behind the Brexit project. Leaving the EU showed “no plan and little planning, oodles of PPE tutorial-level plausible bullshit [and] supreme self-confidence that we understand others’ real interests better than they do”, he said.

He also predicted the UK would face a “cliff edge” in the post-Brexit talks on a trading agreement, sharing the widely held view that the 21 months scheduled will not be enough.

“The EU will use the prospective cliff edge to force concessions, or to offer a thinner deal, more skewed to its interests, in the hope that the UK is desperate enough, pre-election, to get it done,” Rogers said.

Trade talks are due to be completed by the end of 2020, ahead of a general election in 2022.