LONDON — Britain’s grinding negotiations with the European Union for a post-Brexit trade agreement won a reprieve on Sunday, as Prime Minister Boris Johnson and the president of the European Commission, Ursula von der Leyen, agreed to extend the talks despite divisions between the two sides that had looked impossible to bridge.
In a joint statement issued after a midday phone call, Mr. Johnson and Ms. von der Leyen said, “We think it is responsible at this point to go the extra mile.” They characterized the phone call as “useful,” the first glimmer of hope after days in which the negotiations appeared to be heading for the rocks.
Mr. Johnson and Ms. von der Leyen did not set a new deadline for the negotiations, though as a practical matter, the two sides have only until Dec. 31, when the transition period to hammer out a long-term trade agreement expires and Britain and the European Union would impose tariffs on each other’s goods.
A failure to come to terms would thrust Britain into uncharted waters with its largest trading partner. It could lead to disruptions of food shipments, threaten British manufacturing industries, and ignite ugly squabbles between British and French fishing boats over coastal fishing grounds.
Mr. Johnson prepared to deploy four Royal Navy patrol ships to guard British waters after Jan. 1 to defend against foreign incursions. Fishing quotas are one of the main areas of dispute between Britain and the European Union in talks that have dragged on for 11 months without a breakthrough.
A visit by Mr. Johnson to Brussels last Wednesday did nothing to break the impasse and instead served to underline how far apart the two sides are. Mr. Johnson and the president of the European Commission, Ursula von der Leyen, met for dinner and acknowledged afterward that a no-deal outcome was now likely.
Europe’s two most powerful leaders, Chancellor Angela Merkel of Germany and President Emmanuel Macron of France, both refused to engage directly with Mr. Johnson, effectively denying him the opportunity to exploit any divisions between the 27 members of the European Union.
With the odds of failure escalating, London and Brussels engaged in a mixture of finger-pointing and contingency planning. Mr. Johnson met with Michael Gove, the British minister in charge of preparing for a no-deal Brexit. Among the plans is the deployment of Navy patrol ships to halt foreign vessels attempting to enter the exclusive economic zone that extends 200 miles from the British coast.
The prospect of a military confrontation between British and French ships on the high seas provoked alarm and fierce criticism in Britain, even among members of the Conservative Party establishment.
“This isn’t Elizabethan times anymore; this is global Britain,” Tobias Ellwood, chairman of the defense committee in the House of Commons, said to the BBC. “We need to be raising the bar much higher than this.” A failure to reach a trade deal, Mr. Ellwood said, “would be a retrograde step, a failure of statecraft.”
Chris Patten, a former chairman of the Conservative Party and governor of Hong Kong from 1992 to 1997, accused Mr. Johnson of being on a “runaway train of English exceptionalism.” The prime minister, he added, was “not a Conservative,” committed to alliances, institutions or the rule of law, but an “English nationalist.”
Analysts said they had not given up hope of a last-minute agreement. Mr. Johnson and his advisers would still prefer a deal, as would the leaders of the European Union. Sunday, they noted, was just the latest of multiple deadlines set by the two sides. The talks could easily stretch all the way until New Year’s Eve.
Still, Britain’s strategy to wait until the end of the negotiating period and then push for major concessions appears to have backfired. European negotiators, driven by the French, have been firm on the issue of fishing rights, as well as on another contentious area: state aid to industry and competition rules.
Mr. Johnson has framed Britain’s campaign as an assertion of its sovereignty after leaving the European Union. But diplomats pointed out that European officials held a similarly strong point of principle: defending the integrity of the single market from a new competitor that sits on its doorstep.
“What the U.K. never understood is that the European Union is a political project,” said Kim Darroch, who served as Britain’s permanent representative to the European Union and, later, as ambassador to Washington. “They are going to make decisions based on political, not economic, considerations.”