It is thought that the small bunker in Craigielands Forest, near Moffat, was used as an operational base for an Auxilliary Unit, a secret branch of the Home Guard, often known as “Churchill’s secret army”.

These specially trained teams were often made up of local estate workers who knew the land like the back of their hand.

FLS archaeologist Matt Ritchie said: “This discovery gives us an insight into one of the most secretive units that were operating during WW2.

“It’s quite rare to find these bunkers as their locations were always kept secret – most were buried or lost.

“From records, we know that around seven men used this bunker and at the time were armed with revolvers, Sten guns submachine guns, a sniper’s rifle and explosives.”

Auxilliary Units had a nickname of “scallywags” and were given orders to fight to the death. When the Units were stood down many joined the SAS or other special forces for D Day and served with distinction.

This particular bunker was found by FLS survey technician Kit Rodger. Describing looking for the site he said: “The bunker was missing from our records but as a child I used to play in these woods and visit the bunker so I knew it was there somewhere.

“It was 40 years ago so I only had vague memories of the location and the vicinity had changed a lot and was overgrown with bracken. However, I stumbled across a shallow trench and this led to the bunker door.”

The recently discovered bunker was built to a standard design and was accessed via a hatch at the end of a narrow passage. A second escape hatch was reached at the end of a ladder leading from the other end of the bunker.

The bunker space was formed by an arch of riveted corrugated iron sheets over a cement floor measuring only 7 metres in length by 3 metres wide. It would have contained bunk beds, a table and cooking stove and all the equipment needed.

None of the materials survived, although broken timbers found on the floor may be the remains of the original timber bed frames.

Unfortunately, due to the rarity and importance of this site and for health and safety reasons, it is not open to the public and its precise location will remain a secret.

FLS believe that because of the subterranean location of the bunker, it might be a tempting spot for roosting bats so bat boxes have been installed.