Britain is being led to a no-deal Brexit by a political elite “which has great difficulties discerning and telling the truth”, the UK’s former ambassador to the EU has said, in a withering assessment of the Conservative candidates vying to be prime minister.

Ivan Rogers, who resigned from his post in 2017 after clashing with Theresa May’s senior advisers, suggested it was “probable” the UK would leave the EU without a deal in what he described as an act of “economic lunacy”.

He said the Tory leadership election brought to mind the quote of the French statesman Charles-Maurice de Talleyrand-Périgord about the Bourbons in that they had “learned nothing and forgotten nothing”.

“We hear all manner of undeliverable promises, made as if the events of the last three years have made no impact on the collective consciousness,” Rogers said of the remaining leadership candidates.

Boris Johnson, the clear frontrunner to be the next prime minister, has said the UK will leave the bloc on 31 October with or without an agreement with the 27 EU member states.

Rogers said Johnson’s stance, and the inevitable EU rebuff to a time-limit on the Irish backstop or an alternative arrangement, suggested many were underplaying the risk of Britain crashing out.

There is a widespread perception in EU capitals that a general election is “coming really rather soon – and much sooner than anyone is prepared or incentivised to admit”, Rogers said of Johnson’s likely response to parliament seeking to block a no-deal Brexit.

“This is why serious players in Brussels and certain capitals have concluded, as have I, that we are now rather likely to be headed for a breakdown to ‘no deal’,” he said in his latest speech on Brexit.

“We all know this is a great country. Sadly, it’s one currently very poorly led by a political elite, some masquerading as non-elite, which has great difficulties discerning and telling the truth. I am discouraged by just how badly Brexit has been handled to date, and currently pessimistic that this is going to get any better any time soon.”

On Tuesday, Germany’s EU affairs minister, Michael Roth, speaking before a meeting of ministers from the 28 member states in in Luxembourg, reiterated the EU’s refusal to countenance a renegotiation of the withdrawal agreement once the UK has a new prime minister.

“I don’t see any chances to renegotiate the package,” Roth said. “The withdrawal agreement is the withdrawal agreement and I don’t see any appetite to start new negotiations within the EU … The message of the EU is crystal clear on this issue.”

Luxembourg’s prime minister, Xavier Bettel, said he was sure that Johnson, as a former mayor of London, would be pragmatic, but that there was no hope of a renegotiation.

He said: “We have signed an agreement with Theresa May and I really don’t want to misuse this press conference to really say that Theresa May did the best possible job.

“She had a really difficult job, but she did the best possible agreement able to get and this won’t change now there is a new prime minister. The deal is not with the prime minister, the deal has been agreed with the Government, with the UK, with the institutions.”

Rogers, who resigned as the UK’s ambassador to the EU in January 2017, said a no deal was not overly feared in EU capitals and that Johnson as prime minister would likely make a similar error as May by hemming himself in through a speech to party conference soon after arriving in Downing Street.

In 2017, May had told Tory party members that she would take the UK out of the single market and a customs union, two pledges that posed the problem of avoiding a hard border on the island of Ireland and maintaining “frictionless trade”.

Rogers said: “I am worried that the longer the sheer lack of seriousness and honesty, the delusion mongering goes on, the more we imperil our long-term prospects.

“I was in Brussels running the preparation process for the then prime minister’s first European council in October 2016, and able to feel the severe frost – and, as I say, also the total internal solidarity - her speech 10 days previously to the party conference had engendered,” Rogers recalled.

“Well just think ahead to the October European council this year, three years on, which will be the EU debut, and perhaps the final curtain too, for the next PM, just two weeks after a first party conference leader’s speech.

“And do we think whoever wins the leadership contest will, in their first leader’s speech to conference, have set out a subtle, nuanced, principled and collaborative approach to sober up the party faithful for the many difficult compromises ahead?”