The concrete bunker at Mill Hill was built in the 1950s

The bunker in Mill Hill is one of four in and around London which would have housed civil servants and administrators.

The concrete building, which stands above ground, has been derelict for 50 years.

On Tuesday English Heritage opened it to the press as part of an ongoing campaign to protect facilities from a time when Britain was "gripped in the fear or prospect of a nuclear conflict".

The Grade II listed bunker, which measures 60 ft by 40 ft, would have been home to a senior civil servant and 50 staff who would have been locked inside for at least two weeks.

The walls of the bunker are three feet thick

Its existence was a secret and many of the fittings and facilities inside, including a kitchen, chemical toilets and a control room, have remained untouched.

The bunker was built in the early 1950s when it was thought a building above ground could withstand a nuclear blast.

The walls are three feet thick, with a five foot thick roof and a single steel door.

But within a few years nuclear weapons had become more powerful and better shelters were built underground and away from London.

English Heritage researcher Wayne Cocroft said: "The thinking was if a bomb landed on central London the shelters would survive the blast and the various governing bodies would be based in the four shelters.

The Cold War period is an incredibly defining part of all our lives English Heritage chief executive Dr Simon Thurley

"It would have been senior civil servants and administrators as opposed to the Cabinet who would have been further afield.

"They are a fascinating part of British history and were built when the country was gripped in the fear or prospect of a nuclear conflict and a Soviet attack."

Of the four London bunkers, those at Cheam and Wanstead have been demolished.

The other at Chislehurst, Kent, has been turned into a �3m house.

Missile shelters

English Heritage is worried that the disposal of military and government facilities will lead to many Cold War installations being destroyed.

It has already protected a former Royal Observer Corp headquarters at York and missile shelters in Greenham Common, west Berkshire.

English Heritage chief executive Dr Simon Thurley said: "The Cold War period is an incredibly defining part of all our lives.

"For many years we had a backdrop of protect and survive, Greenham Common and four-minute warnings.

"These buildings are the physical remains of this and we want to keep some of them."

The Mill Hill bunker will not be opened to the public.