Hundreds of poisonous insects are expected to invade Brent this summer.
And local authorities are urging residents to report any sightings of the dangerous creepy-crawlies.
Oak processionary caterpillars - the larval form of the moth - pose a serious threat to the borough's oak tree population and are covered in toxin-coated barbs, which cause painful and potentially serious symptoms in humans and animals.
The critters turned up in the region two years ago and began breeding in trees, but despite numerous attempts to wipe them out, the species is set to rear its ugly head in the coming months.
Roddie Burgess, from the Forestry Commission, said: "We greatly reduced the potential breeding population last year, but didn't destroy every last nest and caterpillar.
"If it's allowed to get established it will be a problem not only to Britain's much-loved oak trees - and possibly other trees that it can also attack - but also to human health. "
Last summer, specialist teams armed with insecticide and protective suits were tasked with eradicating the leaf-eating pests and managed to destroy more than 700 tennis-ball-sized nests.
But a number survived and there are growing worries that they could spread out of the west and south-west area of London and permanently establish themselves in the UK.
Councillor Irwin Van Colle, lead member for environment at Brent Council, said: "We have a good picture of where the infested trees are, but we're still keen for the public to report any sightings.
"We would stress that people should not try to deal with the nests or caterpillars themselves. Even old, disused nests can be hazardous, because they can be packed with the caterpillars' hairs and shed skin."
The distinctive creatures hatch from eggs laid in silk nests, found on the branches and trunks of infested trees.
Their sharp hairs cause irritation and allergic reactions to humans, which include intensely itchy or painful skin rashes, sore throats, eye problems and breathing difficulties.
Dr Roger Gross, from the London Health Protection Agency, said advice is being issued to local GPs and other health professionals.
He added: "If it becomes established here, as it has in other European countries, a lot of people might get a very itchy skin rash and a few will get more troublesome symptoms such as itchy eyes and breathing problems."
Any ightings should be reported to Forest Research on 01420 22255 or to Brent Council on 020 8937 5050.