Five new cases of bluetongue have been identified, prompting officials to extend control zones in East Sussex and East Anglia.
The number of cases now stands at 50 since the first case was discovered one month ago.
The latest cases were found at a farm in Sevenoaks, Kent.
The East Anglia control zone extends to southwest London, and the Kent/East Sussex zone extends into Wealden, both encircled by a larger protection zone.
It follows an extension of the Kent control zone five days ago into East Sussex after two rams were found to have infection at a farm in Northiam.
'Real concern'
Peter Kendall, president of the National Farmers' Union, said the farming industry was hoping the disease would not spread.
"It's causing real concern because no farmers are able to plan for the rest of the year and beyond."
Bluetongue is a non-contagious virus spread by a midge species, affecting ruminants including sheep and cattle.
It can also affect goats and deer.
The protection zone now stretches across much of England, reaching into North Yorkshire, Cheshire, Wiltshire, Gloucestershire and south to the Isle of Wight.
Deputy chief vet Fred Landeg has said the disease had probably entered the country through midges from northern Europe.
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