Experts are considered putting grey squirrels on contraceptive pills as part of a bid to reduce their numbers and protect the country’s trees.
The unusual solution was devised by researchers who want to prevent the animals from damaging broad-leaved trees and threatening the native red squirrel.
They believe the five-year project could help stop the grey squirrel from reproducing at such a great rate.
Grey squirrels carry a disease called squirrelpox virus, which rarely affects their health but often kills red squirrels
Researchers at the Animal and Plant Health Agency are now working to turn a proven injectable formulation into an oral contraceptive which will be delivered to grey squirrels via food in a species-specific feeding hopper.
The Avon Wildlife Trust, which helps protect wildlife in Bristol and surrounding areas, said damage from grey squirrels often leaves trees vulnerable to disease.
A spokeswoman for the trust said: “Across our 30 nature reserves we have fairly high levels of grey squirrel damage to trees including extensive bark stripping.
“We don’t carry out any species control and so we just have to manage our woodlands to protect the trees as best we can.
“But there’s no doubt that the damage caused does put trees under stress and makes them vulnerable to disease so the problem is there.”
The environmental damage caused by grey squirrels was also mentioned in a House of Lords debate on threats from tree pests and diseases earlier this year in February.
Lord Gardiner of Kimble, parliamentary under secretary of state for rural affairs and biosecurity, said: “With support from Defra, research to develop an oral contraceptive as an effective method for controlling grey squirrel populations is delivering promising initial results.
“Fertility control does have the potential to reduce grey squirrel populations and the spread of the squirrel pox virus.”
Grey squirrels are native to North America and are considered an invasive species in the UK as they are not regulated by natural predators.
They are big in size, have great strength and have the ability to store fat. For this reason they have been displacing red squirrels for decades.
Additionally, intensive bark stripping by non-native grey squirrels creates wounds that pathogens can infect, weaken and kill tree species.
The Earl of Kinnoull, chair of the UK Squirrel Accord and Red Squirrel Survival Trust, said: “The UK Squirrel Accord has commissioned a fertility control project at the Animal and Plant Health Agency.
“It is hoped it will perfect a suitable active substance and hopper delivery method to allow fertility control to shrink grey squirrel numbers significantly, allowing forestry a chance.”