Sightings of a hairy caterpillar, that can make humans and pets very unwell, must be reported this summer but people are being urged not to touch them.

The Forestry Commission says England is entering its ‘greatest risk period’ when oak processionary moth caterpillars emerge before turning into adult moths.

Their appearance between late May and July can pose a risk to the public and their animals because both the creatures and their nests contain long hairs which cause itchy rashes, eye and throat problems and sometimes breathing difficulties.

The caterpillars – say officials – should not be handled or touched ‘under any circumstances’.

Andrew Hoppit, oak processionary moth project manager, added: “As we enter the greatest risk period for oak processionary moth, it’s important that those living in affected areas understand the health risks so that they can be vigilant when enjoying outdoor spaces.”

The oak processionary moth is classified as a tree pest and was first identified in London in 2006, but has since spread in much larger numbers to surrounding counties particularly in the south east of England.

Alongside making people - and sometimes pets - feel very unwell if they come into contact with them, the caterpillars also feed on the leaves of 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: "Our oak trees are an iconic part of our British landscape. Reporting any sightings of oak processionary moth to the Forestry Commission will both minimise the pest’s spread and reduce the damaging impact it poses to 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, will have black heads and bodies which are covered in long white hairs. It is a protein in the hair which will cause itchy rashes, eye, and throat irritations.

If you spot the caterpillar, sightings can be reported via the Commission’s Tree Alert portal. Alternatively call 0300 067 4442.

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 animal owners should contact their vet for help and treatment.