Park-goers in Croydon, Hackney, Lewisham and Tower Hamlets are being warned to keep away from a poisonous caterpillar that can cause toxic shock and violent sickness.

The Forestry Commission has put out a warning to keep away and report any sightings of the hairy larvae of the oak processionary moth.

The fuzzy caterpillars pose a threat to people’s health as any contact with their tiny hairs can cause toxic shock and violent sickness in serious cases. Less severe reactions to the insects include sore throats, itchy eyes and skin and difficulty breathing.

Dog walkers should also remain cautious when walking around parks or near trees as the caterpillars could also harm dogs if they come into contact.

The caterpillars in question are easy to identify as they’re covered in around 65,000 long white hairs and they travel in large clusters to feed on the leaves of oak trees. They can also be identified by their grey body and dark head.

Most likely to be on the move in late spring and early summer, the caterpillars will begin to start building nests soon. These can only be found on the trunks and branches of oak trees, can be recognised by their tear-drop shape and are made of white, silken webbing.

The oak processionary moth is a fairly new pest to England as it was accidentally introduced to the country in 2005, but the population has been steadily increasing and the threat they pose continues to rise.

Craig Harrison, the Forestry Commission’s South-East England Director, said: “The public and those working in green spaces such as tree surgeons and gardeners can help by reporting any suspected OPM sightings.

“However, they should not touch the caterpillars or nests themselves; removal is most safely done by specially trained and equipped pest control experts.”

Dr Deborah Turbitt, London Deputy Director for Health Protection for Public Health England, said: “We strongly advise people not to touch or approach the caterpillars or their nests because of the health risks posed by the hairs. Pets and livestock can also be affected, and should be kept away as well. The Forestry Commission website has pictures to help identify the pest.

“People should see a pharmacist for relief from milder skin or eye irritations following possible OPM contact, or consult a GP or NHS 111 for more-serious reactions. Contact a vet if animals are seriously affected.”

The Forestry Commission advise people to stay away from the caterpillars completely. If you have spotted a suspected nest of oak processionary moth caterpillars you should contact the Forestry Commission straight away so they can remove the caterpillars safely.

They can be emailed at opm@forestrycommission.gov.uk or called on 0300 067 4442 and all sightings can also be reported on their Tree Alert online forum.

For more information on the oak processionary moth and its caterpillars, read this helpful guide made by Forest Research.