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A French fisherman repairs a fishing net at the port of Boulogne-sur-Mer, northern France, on Friday, Oct 15, 2021. France wants more fishing licences from London, but the UK is holding back. (AP Photo/Christophe Ena) - 

The French and British leaders agreed on Sunday (Oct 31) to defuse days of sniping over post-Brexit fishing rights, according to Paris, potentially averting a full-blown trade war that would embroil the entire EU.

President Emmanuel Macron and Prime Minister Boris Johnson met for about 25 minutes on the margins of a G20 summit in Rome, aides said, a day after Johnson complained to EU chief Ursula von der Leyen that French threats to trigger reprisals over the row were "completely unjustified".

They agreed to work on "practical and operational measures" to resolve the dispute in the coming days, Macron's office said.

They were united on the need for a "de-escalation" with concrete action to come "as soon as possible", it said.

There was no immediate comment from Downing Street.

Johnson has been stressing at the G20 that all sides must focus on the bigger picture of climate change as he prepares to host more than 120 world leaders at the COP26 summit from Monday.

But both the UK and French governments had been intensifying their angry rhetoric, and France last week detained a British trawler that was allegedly fishing illegally in its waters.

The two sides have also been at loggerheads over a nuclear submarine alliance involving Australia, Britain and the United States, dubbed AUKUS, that left France in the cold.

France is incensed that Britain and the Channel Islands of Jersey and Guernsey have not issued some French boats licences to fish in their waters since Brexit took full effect at the start of 2021.

Paris had vowed that unless more licences are approved, it would ban UK boats from unloading their catches at French ports from Tuesday, and even impose checks on all products brought to France from Britain.

On Friday, French Prime Minister Jean Castex said in a leaked letter to von der Leyen that Britain should be shown "it causes more damage to leave the EU than to stay in".

The letter drew a withering response from British officials, and Johnson warned that London could activate a Brexit dispute tool for the first time, exposing all of the EU to potential UK trade measures.

For his part, Macron warned Friday that Britain's "credibility" was on the line, accusing London of ignoring the Brexit trade deal agreed with Brussels after years of tortuous negotiations.

"When you spend years negotiating a treaty and then a few months later you do the opposite of what was decided on the aspects that suit you the least, it is not a big sign of your credibility," he told the Financial Times//CNA