The British trawler kept by French authorities docks at the port in Le Havre, western France, Thursday, Oct. 28, 2021. AP
President Emmanuel Macron said on Sunday that Britain must give ground in the dispute or France will trigger trade reprisals on Tuesday, saying: "The ball is in Britain's court."
But Britain's Foreign Secretary Liz Truss told Sky News on Monday that "we will use the mechanisms of our trade agreement with the EU to take action" if Macron goes ahead with his plans.
"The French have made completely unreasonable threats, including to the Channel Islands and to our fishing industry, and they need to withdraw those threats," she said.
"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."
France is enraged that Britain and the Channel Islands of Jersey and Guernsey have not issued some French boats with licences to fish in their waters since Brexit took full effect at the start of 2021.
Paris has vowed that unless licences are approved, it will ban UK boats from unloading their catches at French ports from Tuesday and impose checks on all products brought to France from Britain.
Jersey's Minister for External Relations Ian Gorst said on Monday that his officials had followed the terms of the Brexit deal, which stipulates that boats should only be granted licences if evidence is presented that they fished in the waters before 2016.
"We've really tumbled over ourselves to be as engaging and as reasonable as possible," he told Sky News.
"And so to hear that tomorrow, the French will take countermeasures, it's extremely frustrating. I mean, it's bordering on they're just being silly."
The simmering feud over fish has already seen a British trawler detained in a French port and Paris's ambassador in London summoned to the Foreign Office for the type of dressing down usually reserved for hostile states not allies.
Britain's Prime Minister Boris Johnson complained to EU chief Ursula Von Der Leyen on Saturday that French threats over fishing were "completely unjustified" as he held out the option of invoking the Brexit dispute tool for the first time, drawing the EU itself into the row.
"If there is a (French) breach of the treaty or we think there is a breach of the treaty, then we will do what is necessary to protect British interests," he told Sky News.