Churchill's last refuge - so secret even the King wasn't told

It would be all too easy to miss the small, unremarkable door set in a wall between two semi-detached homes on a neat suburban housing estate.

Behind it, however, lies one of the Second World War's most closely guarded secrets, an impregnable labyrinth of concrete chambers designed as a last refuge for Winston Churchill and his cabinet.

Paddock – a replica of the Cabinet War Rooms – the location of which was known only to a select few, has been dormant for half a century. But yesterday the underground complex beneath a housing estate in Neasden, north London, was opened to the public for the first time.

The first few to peek beyond the door and walk its complex of corridors were quick to appreciate the historical significance of the site.

One of them, Mayur Jobanputra, 40, from Stevenage, said: "You could smell the history down there; a really musty, damp smell. On top it looks just like a housing estate but there's this huge bunker beneath it. Everything is still there – the telecoms office, the cafeteria, some Fray Bentos tins, even where Churchill would have kipped at night."

Designed to hold the 200 staff of the war cabinet, Paddock's location 12m (40ft) below ground was so closely guarded that King George VI was not even entrusted with its whereabouts. Churchill simply referred to it as "near Hampstead" in his memoirs.

The Whitehall war rooms were not built to withstand a direct hit, so the new version – covered with steel-reinforced concrete three-and-a-half feet thick – would have been Churchill's last refuge if the Battle of Britain had been lost.

The citadel might have remained secret from the public if it was not for the fact that, a few years ago, the Network housing association secured planning permission to build 37 homes on top of it.

Network will admit visitors during a few specified days a year – the next being 29 April – on tours organised by Subterranea Britannica.