BRUSSELS: British Prime Minister Boris Johnson and EU chief Ursula von der Leyen agreed on Sunday to continue post-Brexit trade talks even though they were past self-determined deadlines.

The couple said they would decide whether or not a deal was possible over the weekend but after a cross-Channel crisis call agreed to “go the extra mile”.

“We had a useful call this morning. We discussed a major topic that has not been resolved,” von der Leyen said in a brief television statement also issued jointly by British officials.

“Our negotiation team has been working day and night for the last few days.”

Michel Barnier from the European Union and David Frost from Britain held talks on Saturday evening and Sunday morning and will continue negotiations in Brussels.

“And despite exhaustion after nearly a year of negotiations, despite the fact that deadlines have been repeatedly missed, we think at this point it is the responsibility to go the extra mile,” the leaders said.

“We have given a mandate to our negotiators to continue the talks and to see if an agreement even at this final stage can be reached.”

Sunday is the latest in a series of supposedly tough deadlines for talks but tensions are escalating with only 19 days remaining until Britain leaves the EU single market.

On Saturday, Britain took the dramatic step by announcing that naval vessels would patrol its waters from January 1 to remove European crews from fishing grounds they had divided, in some cases for centuries.

The tone of Brussels is not overly aggressive, and von der Leyen has made it clear that the EU will respect British sovereignty after the post-Brexit transition period, but neither side is prepared to compromise on core principles.

Without a trade deal, cross-channel trade will return to WTO rules, with tariffs raising prices and producing paperwork for importers, and failed negotiations could poison relations between London and Brussels for years to come.

Irish Prime Minister Micheal Martin told the BBC that 97 per cent of the deal had been negotiated and “I think the remaining three per cent should not be beyond the ability of both parties to bridge”. “The fact that they negotiated late into the night is an important sign in itself. “Where the dialogue continues, it gives me hope,” he said.

Most of the text of the trade deal might be said to be ready, but Britain has rejected Brussels’s insistence on a mechanism that would allow it to retaliate if UK and EU laws deviate in a way that puts continental companies at a competitive disadvantage.

“The single market defense is a red line for the European Union. What we are proposing to the UK is to respect British sovereignty. This could be the basis for a deal, ”said a senior EU source, echoing von der Leyen’s previous statements.

In London, a government spokesman stressed late Saturday that Britain is ready to leave unions and take care of its own affairs after 47 years of close economic integration and that “as the situation is, the offer on the table from the EU remains unacceptable”. “The prime minister will not leave any stone unturned in this process, but he is absolutely clear: any deal must be fair and respect the fundamental position that Britain will become a sovereign nation within three weeks,” the source said.

Downing Street said the government had mapped out “every predictable scenario” for potential problems after December 31, and “no one has to worry about food, medicine or our essential supply chain.”

Meanwhile, Boris Johnson said on Sunday that he still hopes for a post-Brexit trade deal with the European Union but that failure remains the “most likely” outcome.

“I’m afraid we are still very far away on some important things but where life is hopeful,” he told reporters. “The most likely thing now is of course we have to be prepared for WTO requirements.”