The Forestry Commission says a disease that has killed 90% of ash trees in Denmark has been found in the countryside in East Anglia.
Until now, cases of the fungal infection are thought to have been limited to nurseries and newly planted woodlands.
There are now calls for an immediate ban on ash trees from Europe.
"This is serious," Roger Coppock, head of analysts at the Forestry Commission, told Today presenter Evan Davis.
"It is very difficult to deal with a fungal pathogens like this
they can spread 20km or so carried by the wind," he explained.
"We are trying to monitor the extent of this out break at the present time."
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