WATCH: water quality too low
Europe's not getting its mussels from some of the waters off the Devon coast because of issues about water quality in Lyme Bay.
The sea has been classified at level B, not the top grade that would permit imports into the European Union. The shellfish need to be purified if they're going to get into countries like France and Belgium, which consume much higher proportions of mussels than the average person in Britain.
That's a major issue for mussel farmers, and especially John Holmyard, who has invested millions in the UK's largest mussel farm south of Sidmouth.
His family business is now on the brink of collapse.
When the UK was a member of the EU and part of the single market, the water-quality issue didn't arise.
This video report is from BBC Spotlight.