Bartlett Tree Experts area manager Richard Trippett discovered the moth in Pangbourne near Reading last week and warned the pest was spreading, with not enough arborists knowing how to identify it.
He said of the Pangbourne discovery: "It's a case of finding out who planted that tree, where it came from and whether or not it's a new outbreak.
"The plant passport system is obviously not working - if you have trees from Holland or Germany planted in the past two to three years, you need to be getting out there and checking them.
"We are trying to control it and keep it to London, but it looks like it is going elsewhere because people are not aware of it. Tree surgeons in London should be aware of it, but I don't think they all are.
"As well as making the public aware of it, it's a case of letting people in the industry know what's going on."
A Forestry Commission representative said: "We can confirm that we are investigating reports that nests of oak processionary moth have been found in oak trees recently imported and planted in Pangbourne, near Reading in Berkshire, and in Hampton, Surrey. Our surveyors are inspecting trees affected and other trees in the areas for any other signs of the pest."
Any sightings should be reported to Christine Tilbury on email@example.com or to Forest Research on 01420 222555.