Sightings of a hairy caterpillar that can make humans and pets very unwell must be reported but people are being urged not to touch the tree pest.

The Forestry Commission says it is already receiving accounts of oak processionary moth caterpillars causing “painful rashes” because a warm, sunny spring has meant their growth is higher than in previous years.

The caterpillars, predominantly found in the south east, tend to emerge between May and July to feed before turning into adult moths.

But because the creatures and their nests contain long hairs, which cause itchy rashes, eye and throat problems, and sometimes breathing difficulties, they should not be handled or touched 'under any circumstances' says the Commission.

Instead it is asking the public who may see the creatures to report any sightings as the region enters what it calls the “greatest risk period”.

Oak Processionary Moth programme manager for the Commission Dr Edward Straw said: “The warm weather in spring has led to oak processionary moth growing quicker than in previous years.

“We’re already receiving reports of homeowners being affected by the caterpillars, coming out in painful rashes, and it is important those living and working in areas affected by oak processionary moth remain vigilant about the health risks they pose.”

The Oak processionary moth was first identified in London in 2006 after being accidentally transported from Europe in trees destined for planting and has since spread to surrounding counties including Kent.

Alongside making people - and sometimes pets - feel very unwell if they come into contact with them, the caterpillars also feed on several species of oak trees that causes them to lose their leaves, affect their growth and become more vulnerable to other stresses like drought.

Professor Nicola Spence, UK Chief Plant Health Officer, said: “Oak trees are an iconic and much-loved part of our British landscape.

“By reporting any sightings of the oak processionary moth to the Forestry Commission, we can all minimise the pest’s spread as well as reduce their impact on tree health.”

How to identify oak processionary moth caterpillars

Nests of the caterpillar are typically dome or teardrop-shaped and around the size of a tennis ball.

The caterpillars, which are around 2cm long when fully grown, have black heads and bodies which are covered in long white hairs. It's a protein in the hair which will cause itchy rashes, eye, and throat irritations.

Any sightings can be reported to the Forestry Commission via its special portal here, alternatively people can email opm@forestrycommission.gov.uk.

Anyone who thinks they have come into contact with one of the caterpillars should visit a pharmacist for help with milder reactions or consult a GP for more serious reactions.

If animals have been seriously affected owners should contact their vet for help and treatment.