The U.K. fishing industry warned French boats will blockade ports if their access to British waters is reduced after Brexit, something the government has promised to deliver in its trade talks with the European Union.

“As day follows night, there will be blockades,” Barrie Deas, chief executive officer of the National Federation of Fishermen’s Organizations, told reporters on a conference call on Tuesday. “They’ve done it for much less in the past.”

The right to fish around the U.K., which has some of Europe’s most fertile waters, is one of the most contentious elements in the country’s post-Brexit negotiations with the EU. Britain is seeking more control over its waters and wants to reduce the catches of EU fleets, hurting coastal communities in France, Ireland and Spain. The disagreement threatens to prevent the two sides from striking a trade agreement this year.

Deas said the EU’s Common Fisheries Policy quotas—which allocate 84% of English Channel cod to French boats, and 9% to U.K. boats—are unfair to British fishermen and the U.K. is right to seek changes.

“There will be a reaction in France by the fishermen, that’s just the nature of the beast we’re dealing with,” he said. “That’s not a reason not to do what we’re doing.”