EU chief Ursula von der Leyen on Thursday published a back-up plan to protect road and air travel and fishing rights if Britain leaves the union without a trade deal.
The UK left the European Union on January 31 and at the end of this month will leave the bloc's single market and customs union, bringing to an end a half-century of ever closer economic integration.
Negotiators from London and Brussels have been trying to agree a follow-on trade pact that would govern cross-Channel business after the transition period ends, but talks are deadlocked with three weeks to go.
"Negotiations are still ongoing. However, given that the end of the transition is very near, there is no guarantee that if and when an agreement is found, it can enter into force on time," she said.
"Our responsibility is to be prepared for all eventualities, including not having a deal in place with the UK on 1 January 2021. That is why we are coming forward with these measures today."
British Prime Minister Boris Johnson travelled to Brussels on Wednesday for a working dinner with von der Leyen to talk through the logjam, and the leaders agreed to give the negotiators until Sunday to make a breakthrough.
But London is refusing to, in Johnson's view, compromise its newly reclaimed sovereignty by signing up to match future EU regulation.
Several EU members, notably France, have been pushing for von der Leyen's Commission and her negotiator Michel Barnier to take a tougher line, and to publish the contingency plan to show they are ready for "no deal".
The EU Commission described the plan as "a set of targeted contingency measures ensuring basic reciprocal air and road connectivity between the EU and the UK, as well as allowing for the possibility of reciprocal fishing access by EU and UK vessels to each other's waters."
The measures would go into effect on January 1. They would come to an end if a deal is found or after a fixed period.
Basic air transport will continue for six months provided the UK agrees to reciprocate, as will access for road haulage.
The interim fisheries regulation would continue until the end of 2021, but it provides for "continued reciprocal access by EU and UK vessels to each other's waters".
Johnson's government insists it will assume full sovereignty over its waters on January 1.
UK demands EU concessions ahead of 'moment of finality'
Britain on Thursday said talks to secure a post-Brexit trade deal with the European Union will not go beyond this weekend unless there is "substantial movement" from Brussels in key areas.
Prime Minister Boris Johnson and European Commission president Ursula von der Leyen met in Brussels on Thursday, and gave themselves until Sunday to decide whether to keep talking.
Asked if talks could be extended, Foreign Secretary Dominic Raab told BBC television: "I can't rule it out but I think we are rapidly approaching the point now where we need some finality.
"I think it's unlikely but I can't categorically exclude it and of course it depends on the progress made between now and then."
Britain left the EU in January and is in a standstill transition period until December 31 while both sides thrash out the terms of their new trading relationship.
But months of negotiations have been stuck on the extent of EU access to UK fishing grounds and fair competition rules.
Raab, a former Brexit minister in the government of Johnson's predecessor Theresa May, said there had not been enough "pragmatism and flexibility on the EU side".
"We're talking about fisheries, level playing field commitment, the EU's attempt to lock us into their rules. I think we need to see substantial movement," he added.
He later told BBC radio that Sunday's deadline was a "moment of finality" and said: "The EU has to move significantly on both those points because they're points of principle.
"We're not going to be treated in a way that no other country would accept or the EU would not accept."
Britain's chief negotiator David Frost and his EU counterpart Michel Barnier resume talks in Brussels on Thursday in what could be a final push in the years-long Brexit process.
Their meeting comes as von der Leyen published a fallback plan in the event of "no deal", ahead of briefing leaders of the 27 EU member states about the talks.
Failure to secure a deal by the end of December will see Britain revert to World Trade Organization (WTO) rules, with tariffs and quotas, from January 1.