The Marine Conservation Society (MCS) has downgraded mackerel from its list of fish suitable to eat.
The society says that international arguments about quotas mean it is no longer a sustainable choice.
It says that mackerel should now be eaten only occasionally and consumers should instead eat herrings or sardines.
But U.K. fishermen say the downgrading is premature and could be counterproductive.
Mackerel has become increasingly popular in recent years, thanks in part to greater awareness of the health benefits associated with eating oily fish. It has also been endorsed by several celebrity chefs.
