BORIS Johnson’s claims he will get Brexit done are “diplomatic amateurism” and his deal will be even harder to achieve than Theresa May’s, a former ambassador to the EU has warned.

Sir Ivan Rogers, who was the senior civil servant dealing with Brexit after the EU referendum until his resignation in 2017, said the biggest crisis in the UK’s attempts to leave is still a year away.

Arguing the UK has been mired in a “political shambles” since the 2016 vote, Sir Ivan said politicians have been “thinking purely tactically and short-term” and suggested Government ministers may be deliberately lying to the public about the reality facing the country.

In a speech at the University of Glasgow, Sir Ivan rubbished claims by ministers that a trade deal with the EU can be completed next year.

He added talks breaking down before the end of 2020 – when the future trading relationship is supposed to be settled – “is much likelier than people realise”.

Sir Ivan told the Policy Scotland lecture: “The further ‘out’ of the European Union we choose to go, and therefore the further we want to go, the longer it will take to negotiate the necessary agreements.

“This is the first critical point which Government ministers either repeatedly continue to get wrong or choose to mislead the British public about when talking in these weeks about ‘getting Brexit done’.”

Sir Ivan said the Prime Minister’s claims a Brexit deal could be concluded swiftly were “diplomatic amateurism dressed up domestically as boldness and decisiveness” and were actually strengthening the EU’s negotiating position.

“It may indeed work splendidly at home, where understandable public boredom and frustration with endless Brexit agonising of course plays well for it,” he said.

“But meanwhile, the EU side is already – as in late 2016 and early 2017 – methodically getting on designing the sequencing of the new process which will maximise its leverage in the next phase.

“Or indeed the next many phases, as I personally think there are several more to come.”

He explained he believes Johnson’s trade deal will be even harder to negotiate than his predecessor’s because of greater divergence.

Sir Ivan said: “Put crudely, the EU will feel that, in the time available, rather little serious can get done and will think that is no bad thing as it can fully exploit UK desperation to get something over the new line.

“Why not take advantage of yet another Prime Minister who has unwisely boxed himself in?”

Sir Ivan also claimed the right-wing Brexiteers in the Tory party are “pretty confident” the Brexit deal Johnson is publicly advocating – will not be accepted by the EU and will lead to their desired no-deal outcome.

During his speech, of more than 11,000 words in length, the 59-year-old former Permanent Representative of the UK to the EU said the country feels like it’s stuck in “Brexiternity”.

Comparing himself to Charles Dickens’s Ghost of Christmas Past, he said he was “pointing at a future which can still be changed but is highly likely to materialise if the message it brings is not heeded”.