Built 35 feet underneath the site of the former Post Office Research Station in Brook Road, Dollis Hill, the bunker was used as a standby facility if Cabinet offices in Whitehall were deemed unsafe.
Codenamed the Paddock during the war, it was built deep enough to survive a direct hit from a 500lb Nazi bomb.
It contains two storeys with 43 rooms, including a map room, kitchen, and Churchill’s War Cabinet room, where he held a Cabinet meeting on October 3, 1940.
The BBC even had a studio built here so broadcasts could be made.