Routine post-import tests made on susceptible animals revealed the virus on Tuesday.

However the Department for Environment, Food and Rural Affairs (Defra) said today it was 'not hugely worried' about the disease spreading in the town.

A spokesman said: "The bluetongue vaccine has been available in Hertfordshire since April and uptake in the area is high."

A second ram bought from the same premises in France and shipped to East Sussex also tested positive.

Defra said neither animal would be slaughtered, claiming the move would be a 'disproportionate' method of disease control.

Bluetongue is transmitted between animals such as cows and sheep by midges, but the environment department does not believe it is circulating in Hemel Hempstead's midge population.

Defra's spokesman was, however, advising farmers to be vigilant and ensure their animals are vaccinated.

He added: "Vaccination remains the only effective tool to allow farmers to protect their animals, and we continue to urge farmers to vaccinate at the first opportunity."