Ministers have ruled out extending the Brexit process into 2021 amid calls for it to be delayed due to the Covid crisis and the deadlock in trade talks.
Opposition politicians urged the post-Brexit transition period to be extended beyond 31 December, saying it would end the current uncertainty and reduce the chances of a damaging no-deal outcome.
But Transport Secretary Grant Shapps said this would “add fuel to the fire”.
He urged people to prepare for the changes happening anyway on 1 January.
Post-Brexit trade talks are continuing on Monday, in Brussels, after negotiators failed to reach an agreement over the weekend.
The EU’s chief Brexit negotiator Michel Barnier and his UK counterpart David Frost held talks on Sunday, but key issues remain unresolved.
A UK government source told the BBC a deal will not be reached unless there is a “substantial shift” from Brussels and it is understood a decision could come later this week.
The UK has been following EU rules since it left the bloc on 31 January but this will stop on 31 December, when the UK will exit the EU’s internal market and customs union.
The impasse in trade talks, and the disruption at UK borders due to the Covid crisis, has led to renewed calls for the UK to reconsider extending the transition period to allow more time for a deal to be done.
Scottish First Minister Nicola Sturgeon said the spread of a new Covid variant “demands our 100% attention” and suggested it would be “unconscionable” to compound the UK’s problems by leaving without a trade deal.
Labour’s London Mayor Sadiq Khan said fighting the coronavirus pandemic required the “full and undivided efforts of ministers more than ever before”.
“Risking the chaos and uncertainty of a no-deal Brexit was reckless even before the latest surge in Covid cases and the worrying news about this latest strain,” he said.
And Simon Hoare, the MP for North Dorset, called for the “clock to be temporarily stopped” on the talks, saying there was not enough time for Parliament to scrutinise any agreement before the end of the year.
But Labour leader Sir Keir Starmer said ministers’ focus should be on securing a deal and that “further dithering” wouldn’t help the country.
“I think that it would be far better for the government to get a deal over the line, either today, tomorrow or certainly next week,” he said at an event in London.
“This is people’s lives, people’s jobs, people’s businesses,” he said. “They need a deal, expect a deal and a deal is what must happen.”
The government has long ruled out any extension to the Brexit process, insisting that the end of the transition period – when the UK leaves the single market and customs union – is set out in law.
Asked whether it might now reconsider, Transport Secretary Grant Shapps told BBC Breakfast “absolutely not”, saying the date had been known about for a “long time” and changing it would “add fuel to the fire”.
He said it was important that people were ready for the substantial changes to the UK’s trading relationship with the EU happening on 1 January whether there is a deal or not.
“The important thing is that businesses continue to prepare, that individuals are prepared,” he said.
Members of the European Parliament have been meeting to discuss the situation, after warning that time had run out for it to ratify a deal by 31 December.
One potential option, should the two sides reach a deal in the coming days, would be for the European Parliament to approve it in principle by 31 December before completing the formal ratification process early next year.
What happens next with Brexit?
In such an event, short-term measures could potentially be put in place to minimise disruption to cross-channel trade before new legally-binding rules come into force.
Whitehall sources have said that it is increasingly likely that the UK will emerge from the transition period without a free trade agreement with the EU.
This will mean that, from 1 January, both sides will rely on World Trade Organization (WTO) rules to govern exports and imports. Tariffs could be introduced on goods being sold and bought, potentially affecting product prices.
The BBC’s Europe editor Katya Adler said the two sides had agreed on the vast majority of issues and that disputes over fishing rights remained the main stumbling block.
Prime Minister Boris Johnson has promised a return of sovereignty over UK waters after Brexit while the EU insists member states’ fishing fleets must retain some access.