The UK will send two Royal Navy patrol vessels to monitor a protest in waters around Jersey’s main port amid a fishing rights row with France.
PM Boris Johnson said “any blockade would be completely unjustified” as over 100 French fishing boats prepared to sail to the island on Thursday.
It follows a threat by France to cut off Jersey’s electricity over new post-Brexit rules for French fishing boats.
No 10 said sending the Navy vessels was a “precautionary measure”.
The new rules – introduced by the Jersey government under the UK-EU Trade and Cooperation Agreement (TCA) – require French boats to show they have a history of fishing in Jersey’s waters.
But French authorities said “new technical measures” for fishing off the Channel Islands had not been communicated to the EU, rendering them “null and void”.
Mr Johnson pledged his “unwavering support” for the island, a Crown dependency 14 miles (22km) off France in the English Channel.
The prime minister held talks with Jersey’s Chief Minister John Le Fondré and Minister of External Affairs Ian Gorst, and “stressed the urgent need for a de-escalation in tensions” between Jersey and France.
The AFP news agency reported that about 100 French fishing vessels would sail to Jersey’s port on Thursday as part of a protest against the new rules, quoting the head of fisheries for the Normandy region, Dimitri Rogoff.
Mr Rogoff said the vessels would not try to block the port at St Helier and would return to France in the afternoon, AFP reported.
Electricity threat
The threat to cut off Jersey’s electricity supply – 95% of which is delivered by underwater cables from France – was made by French Maritime Minister Annick Girardin.
She told the French parliament on Tuesday that new rules governing access to Channel Islands waters were unacceptable – and that France was “ready to use… retaliatory measures”.
“I am sorry it has come to this [but] we will do so if we have to,” she said.
The Jersey government said fishing permits must “correspond to the previous activity a vessel has carried out in Jersey waters” under the terms of the TCA, and its new system was “in line with the data submitted by the French and EU authorities”.
A spokeswoman said it took French complaints over the terms of the licensing agreement “very seriously” and would respond, but said it had acted in “good faith” setting up the regime.
“The government remains committed to the sustainable management of Jersey waters for the benefit of this and future generations,” she said.
The two Royal Navy River Class offshore patrol vessels being sent “protect the UK’s interests at home and abroad” and are based in Portsmouth, according to the Navy’s website.
They each have a total length of 90.5m, 30mm and 20mm guns, and are crewed by 45 sailors and up to 50 Royal Marines.