Instead of culling the animals - which have threatened Britain's native red squirrel with extinction and caused millions of pounds damage to forests - they have put them on the pill.

Harry Pepper of the Forestry Commission's Alice Holt Research Centre said the new technique is a humane way to control a pest.

"We have tried trapping and poisoning, but thse are not techniques that we really want to use," he said. "And this technique does allow us to control squirrel numbers."

A scientist injects a trapped squirrel

The contraceptive is actually a sperm-based vaccine, which is either eaten by or injected into the squirrel. Four weeks later, a blood sample is then tested for pregnancy.

The aim is to generate an immune reaction which will make female grey squirrels infertile. The population will therefore begin to die out.

Trials of the contraceptive began a year ago. If they prove successful, it will be the first such pill in the world to be developed, and could be used as a prototype to control other species.

Super squirrels

However, not everyone agrees with the use of such a vaccine. One charity, Animal Aid, has called it "racial cleansing of nature".

Mr Pepper recognises that some people objected to the vaccine as "altering the balance of nature".

The grey carries, but is immune to, the parapox virus

"But, if this pays off, it could protect trees and red squirrels without large numbers of greys having to be killed," he said.

Grey squirrels (Sciurus carolinensis) were brought over to England from North America 130 years ago.

Since then, they have become a pest, with an estimated population of 2.5 million in England, Wales and central Scotland.

The population rapidly displaced the native red squirrel (Sciurus vulgaris), which once had a population of five million but which now number about 150,000 and which scientists say could be wiped out within a decade.

The red cannot compete for food with the grey, and is also highly susceptible to the fatal parapox virus, which the grey carries but to which it is immune.

Campaigns are continuing across the country to breed more reds.

In May, at the Owl Sanctuary in Ringwood, Hampshire, four "super squirrels" from Belgium - reds which are resistant to the parapox disease - produced young.

A rope bridge has been built across the B852 on the banks of Loch Ness to help red squirrels to get from one side to the other without running the risk of being killed by passing traffic.

And earlier this year, there were confirmed reports of red squirrels in RSPB reserves at Lake Vyrnwy and at Cwm Mynach, in Gwynedd, mid-Wales, where they had previously been thought extinct.