Published: 2:21 PM November 26, 2019 Updated: 6:09 PM September 17, 2020

Former UK permanent representative to the EU Sir Ivan Rogers, shown here in 2017, says that the worst is yet to come for Boris Johnson's Brexit. Picture: Dominic Lipinski/PA Archive/PA Images - Credit: PA Archive/PA Images

Boris Johnson is weakening his negotiating hand with the EU for short-term popularity with British voters - but his trade deal with the EU will be even harder to achieve than Theresa May's, warned a former ambassador to the bloc.

Sir Ivan Rogers, who resigned in 2017 as the senior civil servant dealing with Brexit after the EU referendum, said the prime minister is indulging in "diplomatic amateurism" with his approach to the deal.

In fact, the biggest Brexit crisis is still to come, said Sir Ivan.

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 castigated the government for misleading the public about the scale of the task ahead.

Boris Johnson's ebullient and misleading tactics may play well domestically, but he may well actually be strengthening the EU's negotiating hand in the process, argued the former ambassador.

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In a speech to Policy Scotland at the University of Glasgow, Sir Ivan said that the chance of trade talks breaking down before the end of 2020 "is much likelier than people realise".

Sir Ivan said: "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 that the prime minister's claims that a Brexit deal could be achieevd within 11 months are "diplomatic amateurism dressed up domestically as boldness and decisiveness".

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

"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 because the EU will be able to exploit "UK desperation" for a deal.

Speaking about the likely EU mindset, he said: "Why not take advantage of yet another Prime Minister who has unwisely boxed himself in?"

Responding, Liberal Democrat Brexit spokesperson Tom Brake said: "Boris Johnson's promise to get Brexit done has been exposed as the sham it is.

"His plan would either lead to a catastrophic no deal in 2020 or years more chaos and damaging uncertainty.

"We can still prevent an economically ruinous Brexit that would damage opportunities for future generations.

"This election is a chance to deprive Boris Johnson of a majority so we can secure a brighter future in the EU."