BRUSSELS: European doubts over a post-Brexit trade deal with Britain boiled over on Friday with France threatening a veto as tricky negotiations entered what could be their final hours.

As night fell in London, EU and UK negotiators Michel Barnier and David Frost were locked in last-minute debates over fishing rights, fair trade rules and an enforcement mechanism to govern any deal.

But, with time running out for the accord to be ratified before the end of the year and ahead of Britain's departure from the EU single market, EU capitals are getting cold feet.

"If there's a deal that isn't a good one, we'd oppose it," France's minister for European affairs Clement Beaune told Europe 1 radio, adding that "every country has the right to veto".

A European diplomat told AFP that Belgium, the Netherlands, Spain and Denmark share France's concerns that in the rush to conclude a deal, Barnier will give too much ground on rules to maintain fair competition.

Britain's nearest neighbours suspect Germany and the European Commission are too keen to agree a deal to avoid damaging economic fallout.

"We don't want to lock in an unbalanced relationship for decades to come," the diplomat said.

"We are not going to want to explain to our companies why they are being undercut in their market by enterprising British corporates in a less regulated environment."

Germany currently holds the EU's rotating presidency and is also the bloc's biggest economy and most influential member. Asked about the state of the talks, Chancellor Angela Merkel's spokesman said Europe "is ready to reach an agreement with Great Britain, but not at any price".

"It's clear that there are red lines, yet there is always room for compromise," Steffen Seibert told reporters.

Thus far, the capitals have remained united behind Barnier, who has been battling Frost as UK Prime Minister Boris Johnson faces his own choice about whether to compromise.

"Time is in very short supply. We are at a difficult point in the talks," Johnson's official spokesman told reporters.

"What is certain is that we will not be able to agree a deal that does not respect our fundamental principles on sovereignty and taking back control."

A European source with knowledge of the talks said reports of an imminent conclusion to the eight months of wrangling were "premature", suggesting big differences remained.