The UK sent two Royal Navy patrol vessels to monitor a potential blockade of Jersey's main port after France threatened to cut off power to the Channel Island over post-Brexit fishing conditions.
In a statement on Wednesday evening, Downing Street said Prime Minister Boris Johnson had expressed his "unwavering support for Jersey" - a self-governing dependency of the British Crown - in the clash, as he branded any blockade "completely unjustified".
The British prime minister has also "stressed the urgent need for a de-escalation in tensions and for dialogue between Jersey and France on fishing access", No 10 said.
Downing Street said that the UK would be sending the patrol vessels to monitor the situation as a "precautionary measure".
France's minister for European affairs, Clément Beaune, said on Thursday morning that "these manoeuvres do not impress us".
"I have spoken with David Frost, the British minister responsible for relations with the European Union. Our desire is not to maintain tensions but to have a rapid and complete application of the agreement. Nothing but the agreement and the whole agreement," he added.
France also dispatched two patrol vessels on Thursday morning as a fleet of about 50 to 60 French fishermen headed towards Jersey waters to protest. The boats started heading back towards France in the early afternoon.
The escalation of tensions comes after the French government warned that it could take retaliatory measures over new conditions imposed on French fishermen regarding access to waters around the island of Jersey, which is closer to France than the UK.
Speaking to Parliament on Tuesday, French Minister for the Sea, Annick Girardin, alluded that such measures could involve the "submarine cable transmission of electricity" that supplies the island from France.
She had previously said that the government considers the new requirements for fishing around Jersey — on fishing zones, the number of days fishermen can spend in these waters and what gear they're allowed to fish with — "null and void".
The latest row erupted last Friday after Jersey published a list of 41 vessels equipped with Vessel Monitoring Systems (VMS) and authorised to fish in Jersey's waters since Saturday.
This list is accompanied by new requirements "which have not been agreed, discussed or notified beforehand," the French Ministry of the Sea said then.
"We consider that if new requirements for sea areas or fishing gear are integrated into the licences, even though they have not been notified to the European Commission, they are null and void," the ministry added.
A spokesperson for the British government said on Wednesday afternoon that the threat is "unacceptable and disproportionate" and the UK "trusts France to use the mechanisms" provided for in the post-Brexit agreement "to resolve the issues."
Jersey's external affairs minister, Senator Ian Gorst told the BBC earlier in the day that it "is not the first threat that the French have made to either Jersey or the United Kingdom since we are into this new deal."
"It would seem disproportionate to cut off electricity for the sake of needing to provide extra details so that we can refine the licenses," he added.
In a statement, the island's government had confirmed receiving complaints about the conditions from Normandy, France and the EU. "Such complaints are taken very seriously, and the Government will respond in full."
"The Government remains committed to the sustainable management of Jersey waters for the benefit of this and future generations," it went on.
Girardin said earlier in the week that negotiations were ongoing to obtain more fishing licences while a European Commission spokesperson had stressed that "any conditions" had to be notified with sufficient notice for the other party to "comment or adapt".
"Furthermore, such conditions cannot discriminate against our fishermen," they said, confirming that the Commission would contact the UK authorities on "any specific issue" related to this.
The regional fisheries committees of Brittany and Normandy have meanwhile expressed their "anger and incomprehension" over the British conditions in a joint statement.
They said that they "do not recognise" them because they "are in total violation of the provisions of the Treaty".
"We therefore call for a suspension of all economic relations with Jersey, including the ferry link," it added.
Fishermen in Normandy, who gathered in Cherbourg and Granville on Monday to protest the new British conditions, have also called for retaliatory measures.
"It is obvious that there will be responses to the aggression we have been subjected to by the Jersey authorities in relation to fishing authorisations. We hope that the state will take retaliatory measures," Dimitri Rogoff, president of the Normandy regional fisheries committee, said.