A caterpillar that can make humans and pets very unwell should be reported but people are being urged not to touch them.

The Forestry Commission is appealing to the public to report sightings of oak processionary moth caterpillars, which emerge between May and July before turning into adult moths.

But because the creatures and their nests contain long hairs which cause itchy rashes and 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 of the caterpillar as England enters what it calls 'its greatest risk period'.

The Oak processionary moth, which is a tree pest, was first identified in London in 2006 and has since spread 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: "Reporting any sightings of oak processionary moth to the Forestry Commission will help to protect people, minimise the pest’s spread and help ensure our precious oak trees remain a much loved feature of the landscape.

"The caterpillars and their nests can pose a health risk to the public because of their hairs. I would urge the public not to touch them, and for pets and livestock to be kept away."

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. They can also occasionally cause breathing difficulties in people and pets, say experts, so should not be touched under any circumstances.

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.

Andrew Hoppit, Oak Processionary Moth Project Manager, said: "At this time of year, many of us enjoy being outdoors in green spaces of all kinds. If you live in London and the surrounding areas, it’s really important for you to be aware of the health risks posed by tree pests like oak processionary moth."

Any sightings can be reported to the Forestry Commission via its special portal here, alternatively people can email opm@forestrycommission.gov.uk or call 0300 067 4442.