Digging in the garden on a sunny afternoon you might expect to find nothing more than a few weeds and maybe a stray rock. But last week, one amazed West Hampstead gardener uncovered something rather bigger lurking just below his lawn – a nuclear bunker.

Chris Bidics from Westbere Road was digging out his lawn in an attempt to level a mound that had always formed an awkward part of the garden in the house his wife grew up in, when he struck a reinforced concrete block with his spade.

An iron trap door was then revealed just two feet under the grass – a trap door which opened into what appears to be a nuclear bunker from the Cold War.

The house, on the site of the old West Hampstead library that was destroyed by a German bomb in the Second World War, was rebuilt and is thought to have been owned by the government until the early 1980s Mr Bidics’ wife’s family bought the property and made it into a family home.

At three metres deep and two by three metres wide, the shelter includes thick cabling that could be used for communications, electric sockets and a light fitting along with a line of pegs on the wall.

“The spooky thing,” said Mr Bidics, “is we didn’t know anything about it. We were just sitting on top of it all this time.

“As far as I can tell I think it was built in the 1950s at the time of the boom of the ‘duck and cover’ era. At the time the government was planning the civil defence strategy and had these observer points around Britain – maybe it could be one of those.”

In a twist that seems too lucky to be true Mr Bidics has been a Cold War history fanatic for years – and says he always harboured a secret hope that one day he would have a bunker of his own.

Born in Hungary in the 1970s, stacks of Cold War-era DVDs line his shelves and his own YouTube channel proudly displays music videos that feature footage of advice on what to do in the case of nuclear fallout.

He said: “I am obsessed about this stuff, interested like a kid would be. I grew up behind the iron curtain so we had all these civil defence drills all the way through my childhood in the early 80s and we had public shelters and things. We were getting ready for the worst.

“I even talked, jokingly, about moving the ground and putting a shipping container under the house to make a bunker – and now I don’t need to. I have my own real bunker. It’s my dream come true.”

Terry Charman, the senior historian at Imperial War Museum, said that in his memory no other family had discovered a full nuclear bunker in the garden of their home before.

“Of course ordinary families could, at great cost, purchase their own nuclear shelters or convert a World War Two shelter into an approximation of a nuclear bunker but this doesn’t seem to be a conversion,” he said.

“I believe it was law in Switzerland that every resident had to have a nuclear bunker or access to one.

“But in Britain I think it is quite unusual,, even for a police station or local government building to have a bunker quite like this one.”