The Forestry Commission has renewed its warning to dog walkers and others to avoid touching the nests of oak processionary moths because of the caterpillars' irritating, microscopic hairs.

Touching the nests where the grubs drop thousands of the hairs can result in nasty skin rashes and eye irritation for people and animals - and, in rare cases, breathing problems.

The caterpillars are considered a pest because they can strip an oak tree bare of its leaves.

A spokesman said: "The caterpillars build their distinctive white, silken, webbing nests and trails in June on the trunks and branches of oak trees, anywhere between ground level and many metres high.

"The nests become discoloured after several days, and harder to see as a result. They can also fall out of trees, creating a hazard to curious children and pets, and grazing livestock."

Dr Deborah Turbitt, of Public Health England, said: "We strongly advise people not to touch or approach the nests or the caterpillars because of the health risks, but to see a pharmacist for relief from milder skin or eye irritations if they do come into contact.

"Consult a GP or NHS 111 for more serious reactions, and contact a vet if pets are badly affected."

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The Forestry Commission wants walkers to report the nests on its website.