The airborne disease that threatens to wipe out Britain's ash trees is proving tougher to combat than originally thought.

Forestry chiefs have been forced to rethink tactics in their fight against the Chalara ash dieback, after finding new infected sites across a huge area of Scotland which was supposed to be safe.

Two years ago a strategy was agreed by the authorities on how to combat the disease.

A buffer zone which ran diagonally across Scotland from the Moray Firth to the Clyde was established in which all infected trees were to be removed. The plan was to create a sheltered area to the west, which covered most of the Highlands and Islands, where any diseased trees would also be extracted.

It was hoped the sheltered area could remain disease-free for up to 20 years, allowing new approaches to be developed.

But Forestry Commission Scotland has now confirmed that it will no longer be issuing Statutory Plant Health Notices requiring the removal of Chalara-infected, recently planted ash trees in the ‘sheltered area’ of north-west Scotland and its associated ‘buffer’.

It follows the confirmation of Chalara ash dieback in the wider environment at a number of sites in the buffer area last summer. But crucially this year infected trees were also discovered in the sheltered area.

These were found at different sites within a series of a dozen 10km squares from central Sutherland to the forests west of Inverness, from the Great Glen to Wester Ross, and Morvern down into Argyll.

This change in approach has the support of the Scottish Tree Health Advisory Group comprised of representatives across the Scottish Government, the Forestry Commission, environmental bodies and the land use and commercial sectors.

The sheltered area and buffer were established in 2013 in a bid to slow the arrival and spread of the pathogen in the wider environment to help protect its high nature conservation value ash woodlands in that area.

Dr Anna Brown, Forestry Commission Scotland’s Head of Tree Health, said: “Evidence available at the time the sheltered area was established suggested ash at this location was least likely to be at risk from airborne spores. Statutory action requiring the removal and destruction of recently planted, infected ash, was therefore one of the principal tools that we could use to try and slow the spread of Chalara.

“Now that the disease has been detected in the wider environment these measures are no longer appropriate.

“However, the fight against Chalara continues with an increasing focus on adaptive actions such as collecting ash seed for future use, surveying ash trees for signs of potential disease tolerance, and monitoring of environmental changes in a sample of high nature conservation value ash woodlands.”

Already underway, the adaptive actions complement existing guidance on preserving the environmental benefits of Chalara-infected woodlands, which advises to keep the ash trees for as long as possible. Infected trees are not felled unless there are public safety concerns, because they can help research into the disease.

In addition, further guidance on managing high nature conservation value ash woodland is in development. She said the Chalara Action Plan for Scotland had been fully revised to reflect the change in circumstances and would be published shortly along with the recently revised action plans for Ramorum on larch trees and Dothistroma or needle blight which affects a range of conifers.