Red squirrels will soon disappear from England unless the government funds a vaccine against squirrelpox, one of the biggest groups set up to protect the species has warned.

Conservationists say the English population of non-native grey squirrels has exploded this year, triggered by warmer winters which enable mating pairs to feed and breed all year round, and estimate that 70% are carrying squirrelpox, a virus which is lethal only to red squirrels.

“We’re facing a huge surge of grey squirrels,” said Robert Benson, founder of Penrith and District Red Squirrel Group, which covers 600 square miles of Cumbria.

“We think they are breeding three or four times a year, and having four or five kits each time, leading to a massive expansion in grey squirrel numbers: 15 or 20 young grey squirrels are moving through the countryside [each year], from each breeding pair.”

Benson founded his group 40 years ago when the first grey squirrel was spotted in the region and recently took on an eighth full-time ranger to try to control the local grey squirrel population, helped by teams of part-time volunteers.

“Red squirrels are already under extreme pressure, because the grey squirrels will out-compete them for feed and for territory,” he said. “We’ve already lost them from every county in mainland England, apart from Cumbria and Northumberland.”

Squirrel protection groups in those regions desperately need help from the government to control the local grey squirrel population, he said. “We are at the coal face. England is under extreme threat, and in due course, Scotland will be threatened in the same way.”

He said his group is just one of the many conservation groups across Cumbria and Northumberland working to make sure that the red squirrel survives: “Unless we can manage to control that grey surge, the chances are, in two or three years, the red population will begin to disappear.”

The government needs to urgently invest in developing a vaccine against squirrelpox, while there is still a “viable population” of red squirrels alive who can benefit. If that doesn’t happen soon, he said, “we won’t have red squirrels in England, and probably in the United Kingdom, because Scotland too will go, in time … Defra [the Department for Environment, Food and Rural Affairs] and Natural England have to take seriously the threat to our red squirrels”.

He added: “If it wasn’t for the work we do, the red would already have disappeared from this part of the world.”

For the past two winters, he has been finding pregnant and lactating female grey squirrels in December, January and February. “We’re seeing it now, in November. They shouldn’t be breeding at this time of year, but they are.”

Grey squirrels have a natural advantage over red squirrels because they find it easier to digest the tannins found in acorns, a widespread food source which reds struggle to eat.

When squirrelpox is present in the local squirrel population too, the disappearance of reds accelerates and they are replaced by grey squirrels up to 25 times faster. “Every time a red gets squirrelpox, it dies, and it’s a slow painful death,” said Benson.

He said grey squirrels also cause a “huge amount of damage”, especially to growing timber, and feed on song birds and their eggs.

“They cause damage to property too, by getting into lofts and outhouses, chewing through plumbing and electrics. Yet I’m afraid that the conservation of red squirrels seems to be a low priority for the government.”

Defra and Natural England were contacted for comment.