People are being warned not to touch an invasive species of moth caterpillar which can cause skin rashes and irritation to eyes and throats.
The Oak Processionary is a native of southern Europe, thought to have arrived in the UK as eggs laid on young oak trees imported from the Continent.
Caterpillars damage oak trees by feeding on the leaves, sometimes causing severe leaf loss and leaving the trees vulnerable to other threats.
The Forestry Commission has confirmed that the caterpillars are now emerging on oak trees in England and cautioned the public against coming into contact with them.
Measures are being taken to contain the outbreak in several London boroughs, and to eradicate a small outbreak in Pangbourne, near Reading in West Berkshire.
Mark Parsons, Head of Moth Conservation at Butterfly Conservation said: "The caterpillar hairs are an allergen which can cause a reaction in some people.
"The best advice is do not touch or go near any hairy caterpillar, although most species are benign, and some very scarce. If Oak Processionary is suspected contact the Forestry Commission."