People are being warned not to touch an invasive species of moth caterpillar which can cause skin rashes and irritation to eyes and throats.
Residents in west and south London and Pangbourne in Berkshire have been advised not to touch the oak processionary moths, which are now emerging in oak trees in the areas, and to keep children, pets and livestock away from them.
The oak processionary moth damages oak trees by feeding on the leaves, sometimes causing severe leaf loss and leaving the trees vulnerable to other threats.
The caterpillars' toxic hairs can cause itchy skin rashes in people and animals, as well as eye and throat irritation.
They pose a biggest health risk in May and June when they are in the final stage of development, when their hairs can be blown in the wind or left in the web-like nests which they build in oak trees.
The species, a native of southern Europe, is thought to have arrived in the UK as eggs laid on young oak trees imported from the Continent.
The Forestry Commission is hoping to eradicate the small outbreak in Pangbourne, but it has not proved possible to get rid of the moths from London, where they were first detected in Ealing and Richmond in 2006.
But by establishing a six-mile "buffer" zone around the core outbreak area of west London, where Forestry Commission inspectors will survey for infestations and take steps to ensure they are tackled, it is hoped the spread of the moth can be slowed or prevented.
Alison Field, south east England and London area director for the Forestry Commission, said: "We welcome reports of caterpillars or their nests from the public or others, such as gardeners and tree surgeons, who are out and about in areas with oak trees.
"However, the public should not try to remove the caterpillars or nests themselves. This task needs to be carefully timed to be most effective, and is best done by specially-trained and equipped operators."
Dr Brian McCloskey, director of the Health Protection Agency in London, said: "We strongly advise people not to touch or approach the caterpillars or their nests because of the health risks caused by the toxin-containing hairs. Pets can also be affected and should be kept away as well.
"Anyone who experiences an itchy or painful skin rash or a sore throat and irritated eyes after being near oak trees in these areas should consult their GP or NHS Direct."