The French Sea Ministry justified the action saying the detained trawler did not have a fishing licence but Britain's environment minister refuted it, arguing the European Union had granted one.
French authorities have fined two British fishing vessels and “immobilized” one of them overnight in a port as a dispute over fishing licenses between the countries on both sides of the English Channel intensified.
The French Sea Ministry said in a statement that the fines on Thursday resulted from new boat checks that are “part of the tightening of controls in the Channel, in the context of discussions on licenses with the United Kingdom and the European Commission.”
“We are reinforcing the checks,” Minister of the Sea Annick Girardin told French radio network.
“One (vessel) was fined for refusing to let the check take place, and the other one didn’t have the right to fish in the zone because it didn’t have a license.”
Girardin said the vessel was detained during routing checks off the northern port of Le Havre overnight.
However, Britain's Environment minister George Eustice said the trawler seized by France did have a licence.
"They were on the list that was provided by the MMO (Marine Management Organisation) initially to the European Union. The European Union therefore did grant a licence," he told parliament.
France announced on Wednesday that it would bar British fishing boats from some French ports starting next week if no deal is reached with the UK in a dispute over fishing licenses.
It also suggested it might restrict energy supplies to the Channel Islands, British Crown dependencies that lie off the coast of France.
The UK said the threat appeared to breach international law and vowed to retaliate if Paris went through with the move.
“France’s threats are disappointing and disproportionate, and not what we would expect from a close ally and partner,” the UK said.
It said the measures “do not appear to be compatible” with the UK-EU Brexit withdrawal agreement “and wider international law, and, if carried through, will be met with an appropriate and calibrated response.”
France vehemently protested the decision last month by the UK and the Channel Island of Jersey to refuse dozens of French fishing boats licenses to operate in their territorial waters.
France says the restrictions are contrary to the post-Brexit agreement that the British government signed when it left the EU.
After weeks of negotiations, British authorities have issued more fishing licenses but the number still only accounts for 50 percent of what France believes it “is entitled to,” French government spokesman Gabriel Attal said on Wednesday.
Britain says it has granted 98 percent of fishing license applications from European vessels, but there is a dispute over 31 vessels which the UK says did not supply evidence to support their applications.