People across Bromley are being reminded not to touch the nests which toxic oak processionary caterpillars could be building in the borough's oak trees.
They are also being advised to protect animals from contact with the nests, and to report any sightings to the Forestry Commission or local council.
The nests contain thousands of the caterpillars' hairs, which contain an irritating substance which can cause unpleasant skin rashes and eye and throat irritations in people and animals.
They are also a tree pest because they eat oak leaves: large numbers can strip oak trees bare, leaving them weakened and vulnerable to other threats.
The caterpillars are the larval stages of the oak processionary moth (OPM), and in June they build distinctive white, silken, webbing nests and trails on the trunks and branches of oak trees. The nests become discoloured after several days.
There have been three separate outbreaks of the pest since it was first discovered in Britain in 2006: one of which was in the Bromley in 2012, most notably in the grounds of the Royal Bethlem Hospital, Beckenham.
Each outbreak is thought to have begun with an accidental introduction as eggs on oak trees imported from continental Europe.
Ian Gambles, the Forestry Commission's Director England, said: "We want to keep our woods and parks safe for everyone to enjoy, and the public can help us by reporting OPM nests and caterpillars to us or their local council so that they can be properly removed.
"We also advise people against trying to remove the nests themselves, even if they own the oak tree."
Dr Deborah Turbitt, London Deputy Director of Health Protection for Public Health England, added: "We strongly advise people not to touch or approach the caterpillars or their nests because of the health risks.
"Pets can also be affected, and should be kept away as well. The Forestry Commission’s website has pictures to help the public to identify OPM, at forestry.gov.uk/opm
"We advise people to see a pharmacist for relief from milder skin or eye irritations following possible OPM contact, or consult a GP or NHS111 for more-serious reactions. Contact a vet if pets are affected."