The disease is much more difficult to detect in sheep than in other livestock, and government vets have not been able to predict how the disease will spread or how long it will take to control.
Young lambs are particularly susceptible.
At least four large farms were already recording a high mortality rate among lambs, and the disease was spreading through cattle as secondary and even tertiary waves of infection were confirmed, Mr Scudamore said. Sheep farmers had warned of a disaster in the industry if they were not allowed to move pregnant ewes during the lambing season, which starts in the next two weeks. In response, Mr Scudamore announced a limited relaxation of controls on the movement of livestock. Some farmers with pregnant ewes will be allowed to move flocks for up to half a kilometre on their holdings, enabling ewes to give birth under shelter instead of on open ground. He said permitting limited movement would not undermine control measures.