The first case of the tree disease ash dieback has been confirmed in The National Forest.
The disease was found at three sites between Albert Village and Moira, in Leicestershire, near to the Derbyshire town of Swadlincote.
Ash dieback was first identified in the UK last year and is now established in 14 counties around the country.
Millions of trees have been planted at the forest in the last 20 years with ash accounting for about a fifth.
The chair of the National Forest Company (NFC), Catherine Graham-Harrison, said: "We are saddened to learn of the arrival of ash dieback in The National Forest.
"The ash is usually a very robust Midlands tree, prominent in our hedgerows and woodlands.
"We must hope that the more resilient trees survive and breed new generations of trees which withstand the disease."
The outbreak of ash dieback, caused by the the fungus Chalara fraxinea, started after trees were sourced from a nursery in Europe.
About 100,000 trees had to be destroyed in 2012.
Surveys at The National Forest were undertaken by Forestry Commission Plant Health staff who identified the disease on trees that are about 17-years-old, and on some five-year-old trees.
Ms Graham-Harrison said the ash accounted for about 15% to 20% of the trees in the woodland.
However, she added the decayed trees could provide valuable habitats for beetles, fungi and woodpeckers in the future.
The aim of The National Forest project, which started 20 years ago, was to increase the amount of woodland over 200 sq miles (518 sq km) of land in the Midlands, from 6% to about a third.
The area covers the counties of Leicestershire, Derbyshire and Staffordshire, and last year the Duke of Cambridge planted the eight millionth tree in the forest.
"We are only half-way through the forest's creation," said Ms Graham-Harrison.
"Whilst the loss of some of its ash trees will be a big blow, we still have the opportunity to add many more new woodlands to the landscape with other species in the future."