A deadly tree and plant disease first found in the UK in 2002 has spread to Wales, the Forestry Commission says.
Spores of the fungus-like organism referred to as "sudden oak death" have spread across the Bristol Channel to south Wales, said the commission.
In 2009 Japanese larch trees in south west England were found to be infected.
"This appearance and spread into larch trees in Wales add to our concern," said Roddie Burgess of the Forestry Commission.
The organism, Phytophthora ramorum, gets its common name because it kills many of the trees and plants that it infects, the commission explained.
It was first identified eight years ago on a viburnum plant at a garden centre and has since infected shrubs including rhododendrons, viburnums and bilberries.
Last year's outbreak made south west England the only place in the world where it has attacked large numbers of commercially grown conifer species.
The Forestry Commission confirmed the pathogen had spread to south Wales, probably carried as spores in rain, mist and air currents across the Bristol Channel.
The infection has been confirmed in Japanese larch trees in woodland managed by Forestry Commission Wales in the Afan Valley near Port Talbot, in Garw Valley near Bridgend, and the Vale of Glamorgan.
The Forestry Commission expects to find wider infection as it begins ground inspections in areas where aerial surveys have raised suspicions.
Mr Burgess, head of the commission's plant health service, said it and its partner organisations had moved quickly in 2002 to deal with the infection to try to prevent it from spreading.
"Given the damage it has caused elsewhere, we were very concerned when Phytophthora ramorum turned up in Britain," he said.
He said most of the infected trees in south west England had been felled before this year's new needles formed and new spores could be produced.
He added: "Based on our scientists' knowledge of local weather patterns and how it spreads, we remain hopeful that by taking quick action now in Wales as well we might still prevent the infection from the large trees from spreading further north and east outside South Wales and South West England."
The Forestry Commission said it was working with partners to monitor woodlands, fell infected trees and destroy other diseased plants as quickly as possible.
Signs have also been put up in the infected areas asking the visiting public, as well as forestry workers, to takes steps including washing boots, equipment and bike and vehicle wheels to help prevent spreading the disease.
Forestry Commission Wales experts are working to contain an infection in Japanese larch trees in South Wales.
Experts say it is the first time the fungus-like pathogen has been found on larch outside of the outbreak in south west England last year.