The UK has given France 48 hours to back down in a fishing row that could escalate into spiraling legal action under the Brexit trade deal or a wider trade war between two of Europe's biggest economies.

Post-Brexit arguments over fishing rights climaxed last Wednesday when French authorities seized a British scallop dredger near the port of Le Havre. Paris has threatened to impose sanctions on the UK from November 2, which could cause huge backlogs of cross-Channel trade.

The measures may include banning British vessels from some French ports and increased border and sanitary checks on goods from the UK.

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"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," UK Foreign Secretary Liz Truss told Sky News on Monday.

"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 about the time frame the UK government was giving for the French side to back down, Truss said: "This issue needs to be resolved in the next 48 hours."

Britain and France have tussled over rights to the rich fishing grounds around their northern coasts for decades, but a new dispute broke out in September after Paris accused London of failing to give out enough licenses to French boats to fish in UK waters.

The UK government says it is only issuing licenses to ships that can prove they have previously fished in its waters – a central demand from British fishermen, who fear French competition.

France's President Emmanuel Macron and UK Prime Minister Boris Johnson discussed the matter on Sunday on the sidelines of the G20 summit in Rome. However, in spite of an optimistic statement from Paris saying they were close to de-escalating the row, the leaders failed to make headway.

Earlier, Johnson said he was "puzzled" over a letter by the French prime minister reportedly asking for the UK to be punished for leaving the EU, but maintained that the two countries remained "friends."

Relations between London and Paris have grown increasingly tense since Britain voted to leave the EU in 2016, exacerbated by London joining allies Australia and the U.S. in a nuclear submarine agreement that cut Paris out of the deal.

The dispute also risked destabilizing talks during last weekend's G20 summit and this week's United Nations COP26 climate talks in Glasgow.

Pressed on why the fishing row had again soured bilateral ties at such an important time, Truss suggested it may have something to do with next year's presidential election in France.