The Chalara fraxinea fungus, which causes a disease called Chalara dieback, was first found in ash trees in a nursery in Buckinghamshire in February, The Daily Telegraph reported.
Since then, it has spread and been confirmed in woodland in East Anglia.
Environment Secretary Owen Paterson said Saturday a ban on importing ash trees to the United Kingdom will come into effect Monday.
"I have already prepared the legislation and we're ready to go. The evidence is clearly there. It will now be illegal to bring in ash trees and move them around," he said.