A senior EU figure says Boris Johnson will be forced to U-turn on his refusal to extend the post-Brexit transition period beyond the end of 2020, branding it a “stunt”.

Phil Hogan, the trade commissioner, warned the prime minister his “have our cake and eat it” rhetoric would be exposed as false once the hard reality of the negotiations bites in the new year.

And he said the British people were still being shielded from the significant economic damage to come from pulling out of the single market and customs union.

“This move still baffles me because the full consequences of that decision are still not understood in the UK,” Mr Hogan said. “Why trade a Rolls-Royce for a second-hand saloon?”

The jibes set the scene for the punishing talks to come after the UK formally leaves the EU on 31 January – but with the crucial future trading relationship undecided.

Mr Johnson has toughened his stance against an extension to the transition – during which trade will continue uninterrupted – by planning to make it illegal for the government to agree one.

A clause has been inserted into the Withdrawal Agreement Bill, which MPs will debate next week, and is certain to pass given the huge Conservative majority following the election victory.

The move has been criticised as reckless, putting a no-deal Brexit firmly on the table in just a year’s time with the threat of crashing onto inferior World Trade Organisation terms.

But Mr Hogan, interviewed by the Irish Times, said: “In the past, we saw the way the prime minister promised to die in the ditch rather than extend the deadline for Brexit, only for him to do just that.

Bizarre: Boris Johnson drives 'Brexit' digger through boxes Show all 8 1 /8 Bizarre: Boris Johnson drives 'Brexit' digger through boxes Bizarre: Boris Johnson drives 'Brexit' digger through boxes Conservative party leader Boris Johnson drives a JCB through a fake wall POOL/AFP via Getty Images Bizarre: Boris Johnson drives 'Brexit' digger through boxes Prime Minister Boris Johnson during a visit to JCB cab manufacturing centre in Uttoxeter, while on the General Election campaign trail. PA Photo. Picture date: Tuesday December 10, 2019. See PA story POLITICS Election. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative party leader Boris Johnson drives a Union flag-themed JCB, with the words "Get Brexit Done" inside the digger bucket, through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Staffordshire, on December 10, 2019. - Britain will go to the polls on December 12, 2019 to vote in a pre-Christmas general election. (Photo by Ben STANSALL / POOL / AFP) (Photo by BEN STANSALL/POOL/AFP via Getty Images) BEN STANSALL AFP via Getty Bizarre: Boris Johnson drives 'Brexit' digger through boxes Prime Minister Boris Johnson sits in a cab of a JCB during a visit to its cab manufacturing centre in Uttoxeter, while on the General Election campaign trail. PA Photo. Picture date: Tuesday December 10, 2019. See PA story POLITICS Election. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative leader Boris Johnson drives a Union flag-themed JCB, with the words "Get Brexit Done" inside the digger bucket, through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Britain, December 10, 2019. Ben Stansall/AFP/Pool via REUTERS POOL REUTERS Bizarre: Boris Johnson drives 'Brexit' digger through boxes A sign made for a visit by Prime Minister Boris Johnson to the JCB cab manufacturing centre in Uttoxeter, while on the General Election campaign trail. PA Photo. Picture date: Tuesday December 10, 2019. See PA story POLITICS Election. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative party leader Boris Johnson drives a Union flag-themed JCB, with the words "Get Brexit Done" inside the digger bucket, through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Staffordshire, on December 10, 2019. - Britain will go to the polls on December 12, 2019 to vote in a pre-Christmas general election. (Photo by Ben STANSALL / POOL / AFP) (Photo by BEN STANSALL/POOL/AFP via Getty Images) BEN STANSALL AFP via Getty Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative party leader Boris Johnson reacts as he sits in the cab of a drives a Union flag-themed JCB, after driving through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Staffordshire, on December 10, 2019. - Britain will go to the polls on December 12, 2019 to vote in a pre-Christmas general election. (Photo by Ben STANSALL / POOL / AFP) (Photo by BEN STANSALL/POOL/AFP via Getty Images) BEN STANSALL AFP via Getty

“I don’t believe prime minister Johnson will die in the ditch over the timeline for the future relationship either.”

Describing the no-extension clause as “very odd indeed”, Mr Hogan added: “From our point of view it is important that we move from stunt to substance. It would be helpful if the focus was on content rather than timetables.”

The commissioner, who is Irish, also criticised “the false notion that it is possible to make a clean-break Brexit while retaining all the benefits of EU membership”.

“Now that the political deadlock at Westminster is broken the next phase of Brexit needs to be based on realism and hard facts,” he said.

“Any ‘having our cake and eat it’ rhetoric will not fly. Both sides need to proceed calmly and coherently.”

The government has refused to carry out an economic assessment of Mr Johnson’s Brexit proposals, amid independent warnings they will deliver a multibillion hit to the UK.

Mr Hogan said there was no doubt Britain would be worse off, once there were “two markets not a single market”.