Britain threatened on Friday (October 29) to board French fishing boats and France stood by a plan to impose sanctions on British vessels, after a long-running dispute over post-Brexit fishing rights flared this week.
France seized and impounded a British scallop dredger on Wednesday, and has listed measures that would be taken against Britain if London doesn't allow more French trawlers to fish in UK waters.
French officials say the boat's crew failed to prove it was allowed to fish in French territorial waters.
Britain insists they had the correct documentation, and summoned the French ambassador for talks in London on Friday.
The row is part of a wider dispute over post-Brexit trade arrangements, 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.
British officials could also board more French boats, Eustice said.
The prospect of Paris raising energy tariffs to Britain in retaliation also loomed - on Thursday, data showed Britain was importing about 6% of its electricity supply from France.