Defra has classified the River Ouse in the county as potentially infected waters. It can now ban the movement of fish from farms if thought necessary.

The move follows the discovery of a deadly infectious disease at a trout farm in the Yorkshire Dales last month.

Defra said it was a precautionary measure and investigations so far had found no new cases.

But Jonathan Jowett, from the Yorkshire branch of the British Trout Association, said the consequences could be devastating if the disease is found to have spread.

He said: "If we have this lock-down in place for two or three years without trade outside the zone and without some form of help from the government or compensation our businesses will fail."

Up to 50,000 infected fish were slaughtered after the outbreak of viral haemorrhagic septicaemia (VHS).

VHS affects mainly farmed rainbow trout and can cause up to 80% death rate. The disease poses no risk to human health.