A spokeswoman for the Maritime and Coastguard Agency 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.
As this is outside UK waters, we contacted the French coastguard, who sent a French patrol vessel to the area.
We continue to liaise with the French authorities who have been dealing with the situation.
More top news
The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.