SHERWOOD Forest's ancient oaks could be under threat by coal mining at Notts' last operational pit.

Natural England said a decline in the "exceptional" old trees could be accelerated by the ground subsiding and cracking because of deep mining at Thoresby Colliery in Edwinstowe.

The watchdog, set up to protect and improve England's natural environment, spoke out after a change in the law forced UK Coal to apply for planning permission for work it had already started.

The mining is due to take place underneath the Birklands and Bilhaugh Special Area of Conservation (SAC) – home of Sherwood Forest's most famous attraction, the Major Oak.

The area is also a Site of Special Scientific Interest.

A spokeswoman for Natural England said: "It is designated as an ancient forest rich in wildlife, with an exceptional population of old oak trees.

"The site is protected by both UK and European law.

"Natural England believes that part of the planned extension to Thoresby Colliery may have a significant, adverse effect on Birklands and Bilhaugh SAC, and has advised Notts County Council that approval of the application for this part of the planned extension should not be granted.

"Our advice is based on concerns that a significant number of ancient trees within the SAC could be adversely affected by the ground subsiding and cracking as a result of the deep mining operations.

"The impact of this would be likely to unnaturally accelerate and increase the decline of the ancient trees, contrary to the conservation objectives for the site."

Adrian Wilson, of the Friends of Sherwood Forest group, was also critical of the plans.

He said: "I would be very concerned about it. A lot of the trees there are ancient and can just fall down.

"Half a mile into the forest there is a tree that has been cordoned off and paths diverted around it because it is unstable."

Thoresby Colliery has been mined since the 1920s and the current owners UK Coal recently invested £55m for an expansion to allow production to continue for at least the next eight years.

Previously, it was able to do this under what is called a general development order, giving it automatic planning permission.

But after the Conservation of Habitats and Species Regulations 2010 came into effect on April 1, planning permission was needed because of the site's SAC designation.

UK Coal spokesman Stuart Oliver said: "We have been required to get planning consent to extract coal from the deep soft seam where we are now working after a £55m investment to extend its life by at least eight years – and the jobs of the 500 people who work there.

"We want to mine the coal safely, efficiently and with the minimum effect on the environment.

"We have no reason to believe anything we would do over the next eight years at Thoresby will cause any undue concern environmentally in the areas were are mining.

"Obviously the great oaks are of great importance to the area. We recognise that too.

"That's why we have gone the extra mile to satisfy everyone's environmental concerns."

The mine will be 700 metres below the surface.

Notts County Council's planning and licensing committee, which is meeting at County Hall today, was being advised by planning officers to approve the plans.

A report to councillors was being told that here was no evidence to show the effect on trees would be significant and there were no records of damage from historical working.

Surveys showed there were 40 living trees in the area which could be affected by the ground cracking, as a result of mining.

However an assessment concluded that it was unlikely any would be toppled, and even if they were it is predicted to be below the 1% figure which Natural England considers as having a significant effect on the SAC.