Britain has told France to back down within 48 hours in a fishing row that threatens to spiral into a wider trade dispute, or face tortuous legal action under the Brexit trade deal.
France says Britain has refused to grant its fishermen the correct number of licences to operate in UK waters and has said it could impose targeted measures from Tuesday, including tightening some checks on trucks moving between the nations.
Britain says it is issuing licences to vessels that can prove they have previously fished in UK waters.
The row intensified last week when the French seized a British dredger, the Cornelis Gert Jan, in French waters near Le Havre, saying it did not have the required licences, though the boat's owner said it had all the appropriate documents.
"The French have made completely unreasonable threats, including to the Channel Islands and to our fishing industry, and they need to withdraw those threats or else we will use the mechanisms of our trade agreement with the EU to take action," British Foreign Secretary Liz Truss told Sky News.
"The French have behaved unfairly. It's not within the terms of the trade deal. And if somebody behaves unfairly in a trade deal, you're entitled to take action against them and seek some compensatory measures. And that is what we will do if the French don't back down," Truss said.
Asked over what time frame France should back down, Truss said: "This issue needs to be resolved in the next 48 hours."
The row risks being a distraction at the COP26 climate summit which began in Scotland on Sunday.
Paris has said it could ban British fishing boats from unloading in French ports, carry out additional licence checks on British vessels, tighten controls of trucks and reinforce customs and hygiene controls if talks fail.
British Prime Minister Boris Johnson said on Sunday he had been "puzzled" to read a letter from Paris to the European Union.
Sent by French Prime Minister Jean Castex, it called on the bloc to demonstrate there was "more damage to leaving the EU than to remaining there".
The fishing issue dogged Brexit talks for years, not because of its economic importance but because of its political significance.
If not resolved, it could trigger the beginning of dispute measures in the Brexit trade deal as early as this week.
When asked why the fishing issue, a long source of discord between France and Britain, had again soured ties, Truss suggested it might have something to do with next year's presidential election in France.