The first oak tree in Britain to be affected by the deadly disease sudden oak death has been identified in privately-owned woodland in Sussex.

Government inspectors spotted the tree during a routine visit to the woods, near Crawley, where rhododendrons are already diseased.

The discovery has alarmed conservationists, gardeners and landowners already concerned about the impact of the summer drought.

Many trees are ailing because of the shortage of water, which has made it more difficult to spot the symptoms of sudden oak death.

The disease has already wiped out oak and other species in vast areas of California and Oregon, in the United States, and been found in imported shrubs.

Tony Whitbread, head of conservation at the Sussex Wildlife Trust, said recent research suggested European species were less susceptible than American varieties.

He said: "If it does start to get into native oak stock that would be a worry but it seems it might not be as much of a concern as we first thought.

"Many of our woodlands are practically dominated by the oak tree so if we start to get a disease that is killing oaks it would be extremely damaging for our wildlife."

The disease, caused by the fungus Phytophthora ramorum, was found in a red oak, a native North American species, and so far has not spread to British white oaks nearby.

The first sign is usually a dark sap oozing down the trunk. When the outer bark is peeled away mottled areas of dead and discoloured tissue can be seen.

Although no other mature trees have been found with the disease, there have been more than 300 confirmed cases in shrubs such as camelia, lilac and rhododendrons.

Alexander Henderson, assistant head gardener at Sheffield Park garden, near Uckfield, said: "For a garden such as ours with a large collection it is an extreme concern.

"Sheffield Park is covered with trees and rhododendrons so hopefully we won't get it. The whole horticultural world is gearing up for this one."

A Government spokeswoman said the exact location of the infected oak was not being revealed. The tree is expected to be felled in the next few weeks.

She said: "This is the sort of tree the Americans have had problems with. The American oaks are not the same as our oaks so we hope ours will not be as vulnerable."