Shocked Lisa Marie Camps moved into her new house just a few weeks ago and was intrigued by an unmarked manhole cover.

After enlisting the help of local history buffs Lisa decided to take the plunge and open the mysterious entrance.

She found a rubble-filled extensive air raid shelter from the Second World War.

Lisa's garden in Mitcheldean, Gloucestershire, backs onto the former site of British Acoustic Films, a company that left London in 1941 to avoid the Blitz.

BAF made aircraft plotting tables, searchlights and and anti-aircraft technology - which made it integral to the war effort and a target for Nazi bombing raids.

Local historians suspect the subterranean shelter was for the factory's workers - and think it stretches much further underground than previously thought.

Another theory suggests the bunker was used by the Home Guard and was once populated with brave volunteers prepared to serve as Britain's last line of defence.

Lisa said: "We've started trying to dig it out as it would be amazing to show the local kids this unique bit of history.

"I really want this to be a community thing and help teach our children about the Second World War.

"It's very exciting to find this in our garden. I love Second World War history and my mission now is to find out exactly why it's here."

The home was owned by an elderly couple from the 1950s onward.

But the bunker pre-dates the house - and neighbours told Lisa the concrete safe-house was originally made up of two stories with the upper level protruding above ground.

The elderly couple have since died but their daughter-in-law Tina informed Lisa the underground bunker had still been accessible when her husband was a small child.

Tina, whose husband grew up in the house, told Lisa that when her husband was a small boy in the early 1950s, he ventured into the underground bunker with friend.

He found a bayonet and other remnants from the Second World War: shelves filled with mugs, utensils and scraps of ribbon and fabric that may have been used to repair clothes.

Gloucestershire County Council's archeology department said: "It looks as though the house was built after the air-raid shelter was put in for the British Acoustic Films Factory.

"Records show that the Eastern Avenue estate was built in 1949 so I am guessing the shelter was built for the factory workers and was too much hassle to remove when they built the houses.

"Given this, there is a distinct possibility that it will be larger than a domestic shelter and may therefore be more extensive than it looks."

Gerald Cooke, a local historian that runs the Xerox Museum at Mitcheldean Town Hall, one of the team helping Lisa to unearth the secret bunker.

He said: "I don't think we have absolute clarity about its use and purpose. It seems that it was possibly used by the Home Guard.

"But the many decades that have passed since the shelter was in service has left few clues as to its precise purpose.

"Some decades ago it appears that items were found in the shelter. It was in-filled and today we started to clear the rubble inside the building."