The virus is fatal to reds when they become infected

A team including researchers from Newcastle and London's Queen Mary Universities have studied red squirrel populations in Cumbria and Norfolk.

Pox virus is carried by grey squirrels, but does not affect them, but is fatal to the reds if they are infected.

Researchers said the only way to stop the spread is to cull grey squirrels at entry points to red squirrel refuges.

They studied red squirrel populations in Cumbria and Norfolk, both areas that have suffered squirrel pox outbreaks, and compared them with unaffected red squirrel strongholds.

Squirrel refuge

They looked at how far red squirrels had retreated and found the annual distance was 76 sq km in Cumbria and 96 sq km in Norfolk, compared with 3.6 sq km in Italy and 4.4 sq km in Scotland.

The reds' decline was 17 to 25 times higher in Cumbria and Norfolk where the virus was present than in Italy and Scotland where it was not.

They said the only practical and humane way forward is the strategy of red squirrel refuge areas where the entry of grey squirrels could be targeted.

Research associate at Newcastle University's Institute for Research on Environment and Sustainability, Dr Peter Lurz, said: "We are not trying to wipe out the grey squirrel but, as conservationists, we have a duty to look after the red squirrel as it is a protected native species."