Experts want the public to help chart the progress of the speckled wood butterfly, which has seen a 71 per cent increase in range and an 84 per cent boost in numbers in the last 40 years.
As the climate has warmed, the butterfly has spread to colonise East Anglia, the Midlands and much of northern England, wildlife charity Butterfly Conservation said.
It has also become much more widespread in Scotland, where once it was restricted to the mildest areas of the west coast and the Moray Firth.
The speckled wood is aptly named, flying in partially shaded woodlands with dappled sunlight.
As part of the annual Big Butterfly Count, in which members of the public count common butterfly species to help chart their fortunes, the conservation charity is asking people to record the speckled wood in forests.
Scientists are keen to understand why the species is thriving at a time when three quarters of the UK’s butterflies are in decline, with many previously common species showing worrying slumps.
The speckled wood’s spread has been limited in areas where trees have been cut down, conservationists say.
Woodlands are also a key habitat for threatened butterfly species such as pearl-bordered fritillary, wood white and black hairstreak, and common species such as ringlet, brimstone, comma and holly blue.
Butterfly Conservation president Sir David Attenborough said: “The sight of a speckled wood flitting through the dappled sunlight of woodland glade is a memorable high summer spectacle.
“We need more information about how widespread species are faring in this vitally important butterfly habitat.
“Sadly, our woodlands face a multitude of threats from habitat loss to climate change and various tree diseases.”
The naturalist and TV presenter urged people to take part in the Big Butterfly Count, spending 15 minutes counting butterflies in a local wood.
Butterfly Conservation’s Richard Fox said: “Sadly, the speckled wood is one of relatively few butterflies that have fared well over recent decades.
“As well as being a welcome new addition to woodlands and gardens in many parts of the UK, the butterfly provides a fantastic opportunity to study the impact of climate change on our native species.”
The Big Butterfly Count runs until 7 August. People can take part by finding a sunny spot, spending 15 minutes counting the butterflies they see and submitting sightings to www.bigbutterflycount.org