Anticipating that war would include prolonged attacks from the air, military strategists set about planning an underground refuge from which the Cabinet and the Chiefs of Staff could direct operations.
A set of storerooms under a government building in Whitehall was made fit for purpose, and on Sunday, 27 August 1939, one week before the outbreak of World War II, the Cabinet War Rooms became operational. For the next six years, protected from falling bombs by concrete 1.5m (5ft) thick, an army of civil servants, government ministers, military strategists and even, the Prime Minister, Winston Churchill, lived a troglodyte existence, waging a round-the-clock war on Germany.
One of the first rooms to greet visitors today is the Cabinet War Room. "This is the room from which I will lead the war", Winston Churchill declared when he visited it for the first time in May 194O. At first, however, he was not particularly keen on skulking underground, preferring instead to lead from the thick of the action above ground.
On 14 October 1940, a huge bomb damaged 10 Downing Street and fell uncomfortably close to the underground War Rooms. "Pity it wasn't a bit nearer so that we might have tested our defences," Churchill observed nonchalantly.
The next day, though, the War Cabinet met in the underground War Room, and over the next five years held no fewer than 115 further meetings there. However, Churchill continued to insist on being in the thick of the action and, when the bombs started falling, he would exasperate those charged with his protection by hurrying to the roof of the building above to observe the destruction first hand.
For the ordinary men and women who worked here, conditions were cramped and facilities minimal. There was no sewage system, so occupants had to make do with chemical toilets and chamber pots, while washing facilities consisted of buckets and bowls. Vermin, such as rats and mice, were an ever-present nuisance, and the fact that the smoking ban in the workplace lay some 60 years in the future meant that people went about their duties immersed in a fog of cigarette and cigar smoke.
In August 1945, with the war over, the lights in the War Rooms were switched off for the first time in six years, the doors were locked and the huge bunker fell silent.
Since the 1980s, the warren of corridors and cramped rooms has been open to the public. As they have been preserved more or less exactly as they were during the War, you get the eerie sense that the uniformed men and women of the war years have merely nipped out for a tea break and will be back at any moment.
It is a place where time literally stands still - all the clocks are frozen at 4.58pm, the time at which the War Cabinet met here for the first time on 15 October 1940. Yet the hands of one of the clocks have been known to move mysteriously to a different time, and no one has ever discovered why.
Many employees at the War Rooms today complain of feelings of unease, while phantom cigarette smoke has been smelt. Spectral footfall is sometimes heard echoing along the corridors, and the imprint of a military-style boot has occasionally appeared on the freshly waxed floors.
For some of those dedicated men and women who faced the might of Nazi Germany from deep beneath the streets of Westminster, it would seem that the war still goes on.