This could lead to vaccine which, eaten in bait, would inoculate both male and female animals against grey squirrel sperm, rendering them infertile.
The highly fertile greys were introduced from North America and have spread across most of England and Wales.
The smaller, native red squirrel is unable to survive alongside them in broad- leaved forests. Foresters regard greys as pests because they often strip bark from young trees, killing them. They can legally poison them with rat poison.
It will take more than three years to develop a practical contraceptive vaccine. The Sheffield team has been working on a rabbit contraceptive with some success but it is not yet ready for practical use.