Britain threatened on Friday to board French fishing boats and France stood by a plan to impose sanctions on British vessels in a deepening row over post-Brexit fishing rights.

The long-running dispute flared this week after France listed measures that would be taken against Britain if London did not allow more French trawlers to fish in UK waters and then seized a British boat in French territorial waters.

Britain challenged France's explanation that the scallop dredger had no permission to fish in French waters, and hit back by threatening retaliation and summoning the French ambassador to London for talks later on Friday to explain Paris's actions.

The row is part of a wider dispute over post-Brexit trade arrangements between Britain and the European Union which could lead to severe disruptions before Christmas if it spins out of control.

British Environment Secretary George Eustice said London could retaliate if France enforces sanctions including extra customs checks on British goods from Tuesday. The prospect of Paris also raising energy tariffs to Britain also loomed.

"Obviously it's always open to us to always increase the enforcement that we do on French vessels, to board more of them if that's what they're doing to our vessels," Eustice told BBC television.

"There are other administrative things that we can require of vessels," he said.

French Agriculture Minister Julien Denormandie told France 2 TV there had been no progress in the negotiations for more licences to fish in British and said it was right for France to consider sanctions against Britain.

THREATS AND COUNTER-THREATS

The Cornelis Gert Jan, a scallop dredger, was escorted to the northern French port of Le Havre overnight on Wednesday after its crew failed to prove it was allowed to fish in French territorial waters, French officials said.

British officials said it had the correct documentation. The local prosecutor's office said the vessel's skipper will be called to appear before a court in Le Havre in August, 2022.

France says Britain has refused to grant its fishermen the full number of licences to operate in British waters that France says is warranted. Britain says it is issuing licences to vessels that meet its criteria.

France has threatened to ban British fishing boats from unloading in French ports, carry out additional licence checks on British vessels, tighten controls of trucks, reinforce customs and hygiene controls and raise power tariffs.

Seas Minister Annick Girardin has made clear France cannot cut off electricity supplies to Britain as a retaliatory measure but said it could raise tariffs. Britain was importing about 6% of its electricity supply from France on Thursday, data showed.

Eustice said London's focus for now was trying to resolve the issue with the European Commission, the EU executive body, and with France's ambassador to London.

Britain's Brexit minister, David Frost, was due to hold talks in London on Friday with European Commission Vice-President Maros Sefcovic.

Some British officials portray France's defence of its fishermen as an attempt by President Emmanuel Macron to show he is defending their interests before an election in April in which he is expected to seek a new term.

British Prime Minister Boris Johnson can also ill afford to look weak on fishing rights after leading the campaign to leave the EU.

Fishing makes a small contribution to the French and British economies but is a lifeline for some coastal communities.

Reporting by Costas Pitas, Paul Sandle and William Schombergin London; Sarah Meyssonnier in Le Havre; Layli Foroudi, Juliette Jabkhiro in Paris; Writing by Timothy Heritage; Editing by Angus MacSwan

Our Standards: The Thomson Reuters Trust Principles.

By Costas Pitas and Sarah Meyssonnier