Plant health experts are also concerned about the discovery of the agressive ash-killing fungus Chalara fraxinea in the UK.

P ramorum is best known as the infection which causes sudden oak death, and has wiped out large areas of oak throughout the US. It also kills many other plant species, and in the UK has affected larch badly.

The new type of the pathogen, EU2, has beendiscovered in south-west Scotland and Northern Ireland.

Researchers have concluded that the four varieties now known all originated from one type, hundreds of thousands of years ago. Until recent times, they have remained isolated and developed separately. However, modern timber trading and long-distance plant movement could be changing their evolution and worsening the situation as the different varieties come into contact.

"The co-mingling of these lineages increases potential for sexual recombination and possibly the creation of more virulent strains," said the Forestry Commission.

The arrival of existing varieties in a new area is also making P ramorum more difficult to control, said Professor Clive Brasier of the FC’s Forest Research agency.

"It is frustrating in the UK as our efforts to implement effective plant health measures have been under way for some time, and these new pests and pathogens are often effective at evading the measures," he said.

To tackle the issue, UK plant health authorities and "private sector interests" have formed the Tree Health and Plant Biosecurity Action Plan. The EU is also reviewing plant health plans.

The other new plant health threat, Chalara fraxinea, has been discovered in young ash trees in a Leicestershire car park. This followed interception by the Food and Environment Research Agency of diseased saplings sent from the Netherlands to a Buckinghamshire nursery.

The FC says the pathogen has killed 60-90% of ash trees in Denmark and has potential to destroy millions in the UK. It has urged anyone who has received ash trees in the past five years to check their health and report suspicious symptoms.

A pest alert factsheet on the disease can be found at www.forestry.go.uk/ashdieback.