A warning has gone out to walkers to be on the lookout for a dangerous caterpillar.
The Forestry Commission has issued an alert about the toxic caterpillars of the Oak Processionary Moth which can trigger asthma attacks, fevers and severe skin rashes.
They are also dangerous to dogs and other livestock.
Walkers in Ashdown Forest and the North Downs could be at particular risk.
Staff at Kew Gardens have developed rashes after being stung by the poisonous hairs. And the Forestry Commission warns that parts of Surrey and West Sussex are among the areas likely to be housing the dangerous caterpillars.
A similar caterpillar found in pine trees is prevalent in Southern Europe where authorities use helicopters to spray forests and plantations where they are likely to be found.
The Commission warns the caterpillars are a hazard to tree, human and animal health. They have been already active in oak trees in affected areas in London, Surrey and West Berkshire since April and carefully controlled treatment of affected trees with approved pestiicide to kill them while they were still young enough for the treatment to be effective has been completed.
Remaining caterpillars are now large enough and have descended low enough in the oak trees to be recognisable to the untrained eye. They have also finished building distinctive white, silken webbing nests on the trunks and branches of oak trees.
The caterpillars are most easily recognised by their distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which they derive their name, and the fact that they live and feed almost exclusively on oak trees.
They can sometimes be seen processing across the ground between oak trees, and clustering together as they feed on oak leaves.