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The UK is prepared to take legal action against France over the ongoing row about post-Brexit fishing rights, Foreign Secretary Liz Truss has said.
Last month, the UK and Jersey denied permits to dozens of French boats to operate in their waters.
In retaliation, France threatened to block British boats from some of its ports and cut electricity to Jersey.
Ms Truss told the BBC that France was acting "unfairly" in setting a deadline for issuing more fishing permits.
Officials in Paris say that, unless this happens by Tuesday, they will take action.
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France's minister for Europe, Clément Beaune, accused the UK of making a "political choice" by barring "more than 40%" of French boats from UK and Channel Island waters.
But Ms Truss told BBC Radio 4's Today programme that fishing licences had been awarded to French boats "entirely in accordance" with the post-Brexit deal between the EU and UK.
She warned that unless France withdrew its threats, the UK was prepared to "use the dispute resolution mechanism in the trade deal we signed with the EU to take action against the French".
"We're simply not going to roll over in the face of these threats," she added.
Over the next two weeks, Glasgow hosts the most talked-about global climate summit since a landmark environmental deal was struck in Paris back in 2015.
Expectations of a breakthrough may be limited, but the fight against climate change is a reminder of the many issues that unite France, and the wider EU, with post-Brexit UK.
But we don't hear much about those in the news these days. Sausage spats, fish fights, and a possible looming trade war. That, instead, is what dominates the headlines.
So, are these splits real or politically expedient for the opposing sides? Spoiler alert: there's a lot of politics going on here.
Read Katya's blog in full
On Sunday, Boris Johnson and French President Emmanuel Macron met for 30 minutes during the G20 summit but failed to resolved the problem.
The UK government said it was "up to France" to draw back from its threats, while Mr Macron insisted the ball was "in Britain's court".
UK-French tensions were further inflamed on Friday, when a letter emerged from French Prime Minister Jean Castex to European Commission President Ursula von der Leyen suggesting the fishing dispute was an opportunity to show that EU countries risked more damage from leaving the bloc than staying in.
Mr Johnson said he was "puzzled to read a letter from the French prime minister explicitly asking for Britain to be punished for leaving the EU".
Although fishing is a small part of both the British and French economies, it has been a highly sensitive political issue throughout Brexit.
The latest row began after a British trawler was seized by France and another fined during checks off Le Havre on Thursday.