Chalara dieback, caused by a fungus called Chalara fraxinea, was confirmed at two sites in East Anglia.
Until now, the disease had only been recorded in a few nursery specimens.
Ash trees suffering with C. fraxinea have been found across mainland Europe, with Denmark reporting the disease has infected about 90% of its ash trees.
Experts say that if the disease becomes established, then it could have a similar impact on the landscape as Dutch elm disease had in the 1970s.
This outbreak resulted in the death of most mature English elm by the 1980s. Elms have recovered to some extent, but, in some cases, only through careful husbandry.
The East Anglia outbreak was confirmed by plant scientists from the Food and Environment Research Agency (Fera) at the Woodland Trust's Pound Farm woodland in Suffolk, and Norfolk Wildlife Trust's Lower Wood reserve, in Ashwellthorpe.
In a statement, the Trust said that the fungal infection had been found in "mature ancient woodland and woodland creation areas on our estate".
It added: "We are currently carrying out further investigations at other sites."
Visible symptoms include leaf loss and crown dieback in affected trees, and it can lead to tree death.
In Europe, affected trees are not just in woodlands but are also being found in urban trees in parks and gardens, and also nursery trees.
Chalara dieback of ash has been listed as a quarantine pathogen under national emergency measure and the Forestry Commission has produced guidance, including help on how people can identify possible signs of infection.
John Milton, Norfolk Wildlife Trust's head of nature reserves said that "it is likely we will now see further cases".
"Tracking the disease is going to be difficult with the imminent autumnal leaf fall, so the true extent of the disease in the UK may be difficult to establish until the spring," he said.
Experts are urging people to report suspected cases of dieback in order to prevent the spread of the disease to the wider environment becoming established.
A government consultation on whether to ban imports of ash trees in the UK is set to close on Friday, and it is widely expected that legislation will be passed in time for a ban to be in force by mid-November.
A disease that has the potential to devastate the UK's ash tree population has been recorded for the first time in the UK's natural environment.