LONDON — An ugly spat over post-Brexit fishing rights has erupted into a stranger-than-fiction maritime standoff between Britain and France, as naval ships from both countries converged on Thursday in the waters off the island of Jersey, where dozens of French fishing boats were threatening to blockade a port.
Prime Minister Boris Johnson dispatched two British Navy vessels, the H.M.S. Tamar and the H.M.S. Severn, on Wednesday evening as a “precautionary measure,” according to his office.
On Thursday, France deployed two naval patrol boats near Jersey, about 14 miles off the coast of France to “ensure the safety of navigation” as well as the “safety of human life at sea” in case the situation deteriorated, according to a spokeswoman for the French maritime authorities in charge of the English Channel.
The naval deployments escalated a dispute that has simmered for weeks, after French fishing crews accused the local authorities in Jersey of imposing burdensome new requirements to allow them to continue to fish in Jersey’s coastal waters, following Britain’s split with the European Union in January.
Jersey, the largest of the Channel Islands, is not part of the United Kingdom but is a crown dependency, a special status that gives it self-governing rights, including its own legislative assembly, as well as fiscal and legal systems.
Dozens of French fishing boats have massed near the port of St. Helier, the capital of Jersey, threatening to block access to it. A French government official warned earlier this week that France could cut off the power supply to Jersey, which is delivered through underwater cables from France.
The dispute, which flared unexpectedly on the eve of regional elections in Britain, presented Mr. Johnson with a tailor-made opportunity to flex British military muscles in defense of British fishing rights, which were a sticking point throughout the difficult trade negotiations between Britain and the European Union.
“The prime minister underlined his unwavering support for Jersey,” a spokesman for Downing Street told the British news media on Wednesday. Mr. Johnson, the spokesman said, called for a “de-escalation in tensions” and said any blockade would be “completely unjustified.”
Relations between Britain and France had already soured on a range of issues as Britain and the European Union divorced. President Emmanuel Macron of France raised doubts about the efficacy of a coronavirus vaccine developed at the University of Oxford and produced by AstraZeneca, a British-based drugmaker, prompting charges of “vaccine nationalism.”
In December, Mr. Macron briefly cut off access to freight shipments to and from Britain to prevent a fast-spreading variant of the virus that originated in Britain from leaping across the English Channel. The British tabloids pounced.
“Kick in the Baubles,” said a headline in the Sun, suggesting that France was conspiring to ruin the Christmas holiday for people in Britain. “Monsieur Roadblock Gives Way,” said a headline in the Daily Mail after Mr. Macron agreed to lift the ban, subject to a virus testing program for truck drivers.
Fishing was one of the thorniest issues when Britain negotiated its new trade agreement with the European Union, which came into force in January. The deal ended decades during which Britain’s fishing fleet was under the same system as France, with their catches negotiated regularly among the member countries.
Many in Britain’s fishing industry supported Brexit because they believed that for decades, they had been forced to share too much of the fish caught in Britain’s coastal waters with continental crews.
But the agreement sealed by Mr. Johnson and negotiators in Brussels just before Christmas was a disappointment to British fishing communities, who had been promised a “sea of opportunities” by Brexit supporters.
Instead, the increase in annual quotas for British fishing crews was initially modest. And because Britain has left Europe’s single market for goods, British fish and shellfish require more documentation and checks when sent to markets in continental Europe, making them more difficult and expensive to export.
The trade agreement also addressed the complicated issue of fishing around Jersey. The island has the right to impose its own licensing requirements and has left French fishermen complaining of difficulties in receiving the authorization they need to fish in waters they have worked for decades.
Fishing rights have long provoked acute tensions between Britain and its neighbors. From the 1950s to the 1970s, Britain was embroiled in a confrontation with Iceland that became known as the “cod wars.” At its peak, 37 Royal Naval vessels were mobilized to protect British trawlers in disputed waters.
While these clashes have not mutated into broader military conflicts, analyst and diplomats said there was always a risk of accidental escalation. Others said it served to show the loose ends left by the Brexit process.
“This is the kind of old-fashioned dispute that the European Union was created to prevent,” said Simon Fraser, the former top civil servant in Britain’s Foreign Office. “When you leave the European Union, you risk reopening them.”
“It’s also an extraordinarily retrogressive thing to be fighting over fish in the English Channel, at a time when we’re hosting the G-7 summit and trying to talk about a new global role for Britain,” Mr. Fraser said.
Constant Méheut contributed reporting from Paris.