Boris Johnson is on course for a fresh row with Brussels after a top EU official warned the deadline to complete trade talks may need to be delayed.

Downing Street dismissed a warning from Ursula von der Leyen, the new European Commission president, that the transition period may need to be extended beyond December 2020 to resolve the UK’s future relationship with the EU.

Ms von der Leyen, who took on the role from Jean-Claude Juncker, said she was “very concerned” about the limited timeframe for talks, reviving fears of a no-deal Brexit by the end of next year.

“I am very concerned about how little time we have,” she told the French newspaper Les Echos.

“It seems to me that, on both sides, we should seriously consider whether the negotiations are feasible in such a short time.”

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

She added: “I think it would be reasonable to take stock in the middle of the year and, if necessary, agree on an extension to the transition period.”

But a UK government spokesperson said: “The prime minister has been clear that we will not be extending the implementation period.

“Both the EU and the UK committed to agreeing a future partnership by the end of 2020 in the political declaration and have agreed to work with great energy to achieve this.”

The UK is on course to leave the EU on 31 January following Mr Johnson’s decisive election victory but the bulk of negotiations around areas such as trade, fisheries, education and transport can only begin after that date.

Both sides are braced for fresh clashes, as the prime minister has insisted he will not extend the transition period – enshrining the date into the Brexit bill that takes the UK out of the EU next month.

Before the Christmas recess, the prime minister’s Brexit deal passed its first Commons hurdle with a majority 124, paving the way for the legislation to clear parliament by mid-January.

Mr Johnson’s refusal to countenance an extension raises fresh fears of a no-deal Brexit in 2021, if a pact cannot be signed to prevent tariffs and restrictions on goods traded with the EU.