The UK summoned the French ambassador on Thursday over the seizure of a British boat and "threats" made over post-Brexit fishing rights.
France has seized a British trawler fishing in its territorial waters and issued a warning to a second vessel, claiming that they did not have a license.
The two countries are at loggerheads over licensing rules for EU boats seeking to operate in waters around Britain and the Channel Islands.
The new development came hours after France's Prime Minister Jean Castex offered to open talks to resolve the increasingly bitter row.
British Foreign Secretary Liz Truss said Thursday the ambassador Catherine Colonna would be expected to attend the Foreign Office on Friday “to explain the disappointing and disproportionate threats made against the UK and Channel Islands”.
Also, a UK Government spokesman said, “The proposed French actions are unjustified and do not appear to be compatible on the EU’s part with the Trade and Cooperation Agreement (TCA) or wider international law”.
France says Britain has refused to grant its fishermen the full number of licenses to operate in British waters that France says is warranted.
On Wednesday, Paris announced retaliatory measures if there was no progress in its post-Brexit fishing row with London.
It said the measures include time-consuming checks on all products and a ban on UK vessels landing seafood.
Also on Wednesday, French authorities fined two British boats fishing for scallops during checks, with one detained at Le Havre.
"This isn't a war, but it's a fight. French fishermen have rights, a deal was signed and we must implement this deal," Maritime Minister Annick Girardin said.
Over 200 French fishermen are awaiting licenses to ply waters between six and 12 miles from British shores, and particularly around Jersey, French officials say.
Girardin rebutted claims by London that 98 percent of access applications by EU vessels had been approved, asserting the true figure was 90 percent.
"And all the ones without licenses are French, except for one or two Belgians," she said.