Britain's Prime Minister Boris Johnson and EU chief Ursula von der Leyen will decide whether or not to abandon post-Brexit trade talks on Sunday after their latest cross-Channel crisis call.

British negotiator David Frost met his EU counterpart Michel Barnier ahead of the leaders' call for eleventh-hour talks in the European Commission's Berlaymont headquarters in Brussels.

Frost left again after about an hour-and-a-half and returned to the UK diplomatic mission. Downing Street said the leaders are expected to speak after lunchtime and officials from both sides expect an announcement later in the day.  

Sunday is just the latest in a string of supposedly hard deadlines for the negotiations but, with Britain due to leave the EU single market in 19 days, tensions are rising.

On Saturday, Britain took the dramatic step of announcing that armed naval vessels will patrol its waters from January 1 to exclude European crews from the fishing grounds they have shared, in some cases for centuries.

Brussels' tone has been less bellicose, and von der Leyen has made it clear that the EU will respect UK sovereignty after the post-Brexit transition period, but neither side is yet ready to compromise on core principles.

'Close the door' 

Without a trade deal cross-Channel trade will revert to WTO rules, with tariffs driving up prices and generating paperwork for importers, and the failed negotiation may poison relations between London and Brussels for years to come.

On Wednesday, after what von der Leyen described as a "lively and interesting" working supper with Johnson in Brussels failed to find a breakthrough, the EU chief said they had agreed to "come to a decision by the end of the weekend".

But if the talks are to be extended again, it would only be for "for a maximum of a few days", France's Minister for Europe Clement Beaune told the newspaper Journal du Dimanche. "We're already in extra time," he warned.

In London, British Foreign Secretary Dominic Raab told Sky News he would not yet "close the door" on chances that the talks continue "if EU can find its way to treat us like a normal independent country." 

"I think that the EU is concerned that actually Britain might do rather well once we leave the EU and is worried about the competitive advantage even on the normal global rules that apply," he said.

Ireland stands to lose more than most EU members in a no deal future with its larger neighbour outside the bloc, and Irish Prime Minister Micheal Martin told the BBC he was still hopeful of a breakthrough. 

'No stone unturned' 

Martin said 97 percent of an agreement has been negotiated and "it seems to me that the remaining three percent should not be beyond the capacity of both sides to bridge".

"The fact that they negotiated into the night is an important sign in itself. Where the dialogue continues, that gives me hope," he said. 

Much of the text of a possible trade deal is said to be ready, but Britain has rejected Brussels' insistence on a mechanism to allow it to retaliate if UK and EU law diverge in a way that puts continental firms at a competitive disadvantage.    

"The defence of the single market is a red line for the European Union. What we have proposed to the United Kingdom respects British sovereignty. It could be the basis for an agreement," a senior EU source said, echoing an earlier von der Leyen statement.

In London, a government spokesman stressed Britain was ready to leave the union and handle its own affairs after 47 years of close economic integration and that "as things stand, the offer on the table from the EU remains unacceptable. 

"The prime minister will leave no stone unturned in this process, but he is absolutely clear: any agreement must be fair and respect the fundamental position that the UK will be a sovereign nation in three weeks' time," the source said.

On Saturday, Downing Street had said the government had a playbook that "maps out every single foreseeable scenario" for potential problems after December 31, and "no one needs to worry about our food, medicine or vital supply chains".