The major of the French port city of Calais on Thursday told British Prime Minister Boris Johnson to "calm down" and change strategy in dealing with migrants crossing from France.
"I consider Boris Johnson's pronouncements to be a provocation," said Natacha Bouchart, mayor of the town which is a main embarcation point for the small, overcrowded migrant boats seeking to cross the English Channel this summer.
"I say clearly, that it's a declaration of maritime war since it is out of the question for naval ships to enter (French) territorial waters," she told AFPTV.
Britain's right-wing Conservative Party government has looked to increase pressure on France to prevent migrants coming into UK waters.
Interior Minister Priti Patel and French counterpart Gerald Darmanin signed an agreement last month creating a new joint police intelligence unit to combat migrant traffickers, but it is unclear whether this has had any impact.
Britain's interior ministry has also made a formal request for the Royal Navy to help, but campaigners warn any military intervention to stop boats could be dangerous and may face legal challenges.
The Sunday Telegraph newspaper reported that officials were planning for naval and border force vessels to pick up the migrants and take them directly back to northern France.
French officials insist France is doing all it can, with additional security forces deployed, to prevent the crossings. But they acknowledge the numbers have increased sharply this year.
Since January 1, authorities in northern France have recorded around 350 attempts or crossings involving over 4,000 migrants, compared with 203 attempts and 2,294 migrants for the whole of 2019.
The Calais mayor on Thursday said the problem was of Britain's own making.
"If the migrants want to cross (the channel), it is because the British themselves put out the call. They have done so by failing to touch their legislation for 20 years," she said.
"Therefore I appeal to Boris Johnson, saying you urgently need to calm down, and urgently need to change your methods of welcoming and dealing with migrants."
Bouchart also had a message for Darmanin: "Do not give in, because the British don't know how to negotiate. It's only by standing firm that you achieve something."
Johnson on Monday said illegal migrant crossings of the Channel, which have hit record numbers, were "very bad and stupid and dangerous."