Park users are being warned to watch out for potentially dangerous caterpillars nesting on oak trees.
The caterpillars of oak processionary moths are a pest to the beautiful oak trees on Hampstead Heath and other parks.
The distinctive hair on the larvae are also harmful to humans and can cause itching skin rashes, eye irritations and sore throats in people and animals who come into contact with them.
In rare cases they can cause breathing difficulties and severe allergic reactions.
They should not be touched in any circumstances.
The caterpillars were accidentally introduced from mainland Europe and emerge around April every year, eventually turning into moths between May and July.
They move about in or under oak trees in nose-to-tail processions and their tiny hairs contain an irritating substance called thaumetopoein.
The caterpillars can be identified by their nests, which are typically dome or teardrop-shaped, averaging the size of a tennis ball.
The nests are white when new, but quickly become discoloured and harder to see against the bark. They do not nest in leaves.
The City of London Corporation, which manages the Heath, has warned people not to touch or approach them.
They suggest calling NHS 111 or seeing a doctor if someone does have a reaction or consult a vet for badly affected animals.
If you see any nests or caterpillars report them to the main office of the green space you are visiting.
Hampstead Heath: 020 7332 3322
Highgate Wood: 020 8444 6129
Queen's Park: 020 8969 5661
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