During World War II, the basement of what was at the time known as the Office of Works Building in Great George Street was converted into a protected command centre for central government. Construction started in 1938 and was complete in August 1939, just before the declaration of war on Germany. The site includes many rooms largely unchanged since the wartime period, including the Map Room, Cabinet Room, Churchill’s Bedroom and a secure transatlantic telephone in what appeared to be a toilet cubicle.
The building was opened as a museum in the 1980s and is administered by the IWM (Imperial War Museum). It was originally known as the Cabinet War Rooms but renamed in 2010. The site also includes the Churchill Museum which focuses on the wider life of Winston Churchill.
References: Imperial War Museum: Churchill War Rooms