5. Defending London from the air was a priority of war planning
In the summer of 1940, almost 20% of the population of Great Britain lived within the 750 square miles which comprised greater London. Nine million people lived in the area. In the late 1930s, the British government and military planners discussed the means of moving many of these people, mostly women and children, outside of the city (with similar plans developed for other British urban centers, such as Liverpool). When Britain declared war on Germany in September 1939 a blackout, which had been planned and rehearsed at the time of the Munich Crisis, was imposed in British cities and towns and remained in place for the duration of the war. The blackout was a major cause of complaint from the British people, many of whom argued that it was pointless since the German airplanes were often guided by the fires ignited by their predecessors over the cities.
During the pre-war planning and the Phoney War which preceded the invasion of France, the preparation of bomb shelters was for the most part in the hands of local authorities, and some areas suffered from a lack of adequate shelter space. Preparations were made for the relocation of the government despite the negative impact such a move was likely to have on civilian morale. During the first week of German bombings, the government refused to allow the famous London Underground stations to be used as shelters, ordering them locked when air raid sirens sounded over the city. After the second week of bombing the stations were opened as shelters and by mid-September over 150,000 Londoners slept in the Underground stations.