It was a top secret base which saw men trained to fight to the death in the face of enemy combatants.

But like so much of the wartime preparations to ward off a Nazi invasion, following the end the conflict it was forgotten and left to be reclaimed by nature over time.

However, a secret Second World War underground bunker has been unearthed by a forester who literally stumbled into it during tree felling work in the south of Scotland.

The small bunker was discovered by Forestry and Land Scotland (FLS), during work being carried out in Craigielands Forest, near Moffat.

It is thought to have been used as an operational base for an Auxiliary Unit, a secret branch of the Home Guard, often known as “Churchill’s secret army”.

The specially trained teams were to be responsible for sabotage operations in the event of a Nazi invasion, and were often made up of local estate workers who knew the land well.

The bunker was found largely empty, except for an empty tin can and the possible remains of timber bed frames used by those who occupied it more than 75 years ago.

FLS said that, due to the rarity and importance of the site and for health and safety reasons, the bunker will not be opened to the public and its precise location will remain a secret.

However, because of its subterranean location, it is thought that bats might be tempted to roost there and so bat boxes have been installed.

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.”

Auxiliary Units were given the nickname of “scallywags” and were given orders to fight to the death. Hundreds of bunkers were constructed by the British Army as operational bases for the specially trained units.

When the units were stood down many joined the SAS or other special forces for D-Day and served with distinction.

Bunkers were supposed to be destroyed after the war, but many were left to decay instead, and most have been lost.

FLS survey technician Kit Rodger, who made the discovery, 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, discovered 1.3 metres beneath the surface at its deepest point, was surveyed using 3D laser-scanning and photography.

Built to a standard design, it 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 seven metres in length by three 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.

The brick-built ends of the bunker were divided from the main area by a “blast wall” intended to protect against grenades. Ventilation pipes are found at either end of the structure.

A grill has now been installed over the entrance, to prevent anyone finding it from entering