Britain ' s biggest air raid defence test took place when London and 27 other counties in half of England where plunged into darkness for four hours . Lights of all kinds where extinguished or concealed whilst defending planes , anti aircraft guns and Air Raid Precautions organisations co - operated in combating the " raiding " bombers provided by the Royal Air Force . Photo shows : The electric sings of Piccadilly Circus , London , extinguished at 12:30 am as the black - out began . The streaks of light seen here were made by car side - lights , which were permitted . 11 August 1939