The UK has vowed to retaliate against France if Paris goes ahead with a threat to impose sanctions in an escalation of a row over fishing boats, after a French vessel seized a British trawler.
The French government has said it will block British ships from some ports next week if the post-Brexit dispute over fishing licences is not resolved.
Paris went as far as to suggest it would restrict energy supplies to the Channel Islands if no deal was reached, as relations since Brexit further soured.
French European Affairs Minister Clement Beaune said France would now use the language of force, as that appeared to be all Britain understood.
The seized vessel was fishing in French territorial waters without a licence. A second ship was given a verbal warning.
French maritime gendarmes made multiple checks on fishing vessels off the northern French port of Le Havre overnight, the Maritime Ministry said, as France stepped up surveillance during negotiations.
The seized trawler, now under the control of French judicial authorities, had been rerouted to Le Havre under a maritime police escort and was tied up on the port's quayside.
The vessel's captain could face criminal charges, with his catch confiscated, the ministry added.
Downing Street, speaking before the trawler was seized, said the threats did not seem compatible with “international law” and vowed an “appropriate and calibrated response” if France did not back down.
France has been angered by a decision from the UK and Jersey last month to reject dozens of licences for French boats to fish in their waters.
If an agreement over the licences is not reached by Tuesday, France said it would block British boats from some ports and tighten checks on vessels travelling between France and the UK.
The French ministers for Europe and for maritime affairs said they did not exclude measures against energy supplies to Britain in the coming weeks.
“It is very disappointing that France has felt it necessary to make threats late this evening against the UK fishing industry and seemingly traders more broadly,” said UK Brexit Minister David Frost.
“As we have had no formal communication from the French government on this matter we will be seeking urgent clarification of their plans. We will consider what further action is necessary in that light.”
A British government spokeswoman said: “France’s threats are disappointing and disproportionate, and not what we would expect from a close ally and partner.
“The measures being threatened do not appear to be compatible with the Trade and Co-operation Agreement and wider international law, and if carried through, will be met with an appropriate and calibrated response.”
She said Britain would express its concerns to the EU and the French government, and that the UK had granted 98 per cent of licence applications from European boats.
But the dispute continues over 31 vessels for which the UK did not approve licences, saying they did not have supporting evidence for their applications.