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Emmanuel Macron has raised tensions with Britain by warning that the dispute over fishing rights is a test of the UK's global credibility.
The French president spoke out ahead of the G20 summit in Rome, which sets the stage for the COP26 climate summit.
France and the UK are embroiled in a dispute over post-Brexit fishing rights and licences for French boats.
Mr Macron told the Financial Times that UK backpedalling on Brexit commitments "is not a big sign of... credibility".
He was referring to the fishing row and disputes over Northern Ireland.
Mr Macron said: "Make no mistake, it is not just for the Europeans but all of their partners.
"Because 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."
The argument, which began when the UK and Jersey denied fishing licences to dozens of French boats last month, is essentially over how many French fishing boats are allowed to fish in UK waters.
France has said it could stop UK boats landing in its ports if the row over licences is not resolved.
British Prime Minister Boris Johnson said he fears the EU-UK trade agreement may have been breached.
He added that the UK government would do "whatever is necessary to ensure UK interests".
Meanwhile, the BBC's political editor Laura Kuenssberg says she has seen a letter which appears to show the French prime minister Jean Castex appealing to the EU to demonstrate there is "more damage to leaving the EU than to remaining there".
Analysis
At the heart this is about whether a few dozen small French boats get licences for waters around Jersey based on their history fishing the area.
It's a technical issue and should be relatively easily to deal with.
That it's escalated so the French President and the British Prime Minister are weighing in tells us much.
Brexit has meant renegotiating old arrangements.
There's not yet a new equilibrium, but plenty of opportunities for friction. Access to fishing waters is particularly emotive for both sides.
Mr Macron has stressed that he sees fishing alongside the Northern Ireland Protocol as issues that go to the heart of the UK's "credibility", suggesting that the UK is not honouring the deals it has done.
And France in particular is prepared to retaliate if its interests are threatened.
Boris Johnson, by saying he's "puzzled" at France's irritation, seems not to want things to escalate over fishing.
But will the UK grant more licences? The tensions are clear. And it all comes just as the UK and the EU need to find agreement on the far bigger, more difficult issue of the Northern Ireland Protocol.
France was angered by a decision from the UK and Jersey last month to deny fishing licences to French boats, arguing it was a breach of the Brexit deal.
The country then warned it would block British boats from landing their catches in some French ports next week and tighten checks on UK boats and trucks if the dispute was not resolved by Tuesday.
On Friday morning, Environment Secretary George Eustice said if necessary the UK would respond in turn, saying "two can play at that game".
The government also said it was considering launching "dispute settlement proceedings" with the EU if France goes ahead with the "unjustified measures".