A Devon fisherman has claimed his boat was attacked by French fisherman hurling rocks and firing flares.
The incident is said to have taken place on Monday morning in international waters 15 miles off Caen in a dispute over scallop beds.
Anton Bailey, from Brixham, claimed four French boats "threw rocks and tried to ram us".
The Maritime and Coastguard Agency said it had contacted French coastguards, who sent a patrol vessel to the area.
A spokesman said: "It was reported to us on Monday that a number of British fishing vessels were being confronted by French fishing vessels off the coast of Le Havre.
"The French authorities are continuing to monitor the situation."
Other British crewmen said they were surrounded by the French fishermen, who they said tried to damage their gear.
Mr Bailey said: "They were just trying to stop us from our fishing activity.
"They were shooting flares at other vessels, every underhanded thing you could think of.
"I have never seen this before and I have been at sea 20 years."
Jim Portus, from the South Western Fish Producers Organisation, said it was not the first time such an attack had happened in the waters where British and French fisherman fish for scallops.
"The French have a self-imposed closed season for the five months ending on 1 October," he said.
"Their fishermen weren't exploiting the beds over the summer, but the British fishermen were this summer and have done in the past. Irish boats do too. It causes friction with the French."
The Marine Management Organisation confirmed that a number of UK vessels were in an area where they were allowed to fish.
It said a large number of French vessels were in close proximity.
No-one was available for comment from the French fishing authorities.