France warned on Wednesday of EU retaliation against the UK and Jersey for refusing many fishing licences, as trawlermen threatened new protests and even a blockade on cross-Channel exports.
Fishing rights for EU boats in UK waters was a key stumbling block to negotiations for a Brexit trade accord between London and Brussels, and threatened to sink the deal.
But the issue resurfaced after Britain on Tuesday said it would grant only 12 of 47 applications for new licences allowing small EU boats to fish in its territorial waters.
On Wednesday, the self-governing British Crown dependency of Jersey issued 64 full and 31 temporary licences, but refused 75 applications.
French government spokesman Gabriel Attal said the decisions were "totally unacceptable and inadmissible" and "contravene the agreement that was signed in the framework of Brexit", threatening retaliation through the EU.
"We are going to continue and step up our work with the [European] Commission to move forward on this issue, and also to study possible retaliation measures that could be taken if the agreement is not respected," Mr Attal said.
French Maritime Minister Annick Girardin, speaking after a meeting with her country's fishing representatives, called on EU nations to unite in pressuring London to honour its post-Brexit commitments.
"This unwillingness of the British won't involve this question alone," Ms Girardin said, suggesting "technical subjects" including future fishing quotas.
"We see clearly that on all these issues, the British are dragging their feet. I want the licences back."
UK-French relations are already strained, with France accusing Britain of going behind its back to sign a new defence pact that includes providing US-built nuclear-powered submarines to Australia, despite Canberra's earlier agreement with Paris for subs.
At the same time, London is locked in talks with Brussels over implementing post-Brexit trading arrangements for Northern Ireland.
At Boulogne-sur-Mer, France's biggest fishing port, the mood was bitter.
"At first we were disgusted, but more and more we're just resigned and fed up," Jean-Marie Baheu, who was working on his boat Murex, told AFP.
"What will the fishing situation be in three years? That's when we'll judge the consequences of what the English are imposing on us today."
Olivier Lepretre, president of the regional fisheries committee in the northern Hauts-de-France region, said pressure must be put on Britain.
Mr Lepretre suggested the fishermen had the "port of Calais in their sights", raising the threat of a blockade of the key gateway for goods from mainland Europe to Britain.
Jersey, which sets the terms of fishing in its waters under the Brexit deal agreed to last year, says all unlicensed boats must stop fishing in its waters within 30 days, although it will still accept and consider new evidence to support attempts.
"By issuing these licences in the days ahead, we are ensuring the fishing effort in our waters is similar to pre-Brexit," said the Channel island's Environment Minister, John Young.
"Those boats with an economic dependence on Jersey waters, who've fished here regularly before and have demonstrated it, will receive licences."
In refusing licences on Tuesday, the UK government insisted it had pursued a "reasonable approach", issuing nearly 1,700 licences to boats from the EU to fish in its exclusive economic zone, which is defined as between 12 and 200 nautical miles from the coast.
Protests by French trawlers over fishing rights in Jersey erupted this year and threatened to turn into a full-blown naval incident.
As French trawlers steamed towards Saint Helier, London sent two naval patrol boats to monitor the situation, prompting Paris to respond in kind.
To try to calm tempers, a three-month extension was agreed on for EU boats to fish in Jersey waters. That expires this week, although vessels will still be able to operate for the next month.
Meanwhile, neighbouring Channel island Guernsey, which is also responsible for fishing rules in its waters, announced a "roadmap" for issuing licences to French vessels, with the grace period extended until January while decisions are made.
Before Britain left the bloc, boats from EU countries were allowed to fish in British waters subject to quotas.