Greater London's population declined steadily in the decades after the Second World War, from an estimated peak of 8.6 million in 1939 to around 6.8 million in the 1980s. The principal ports for London moved downstream to Felixstowe and Tilbury, with the London Docklands area becoming a focus for regeneration, including the Canary Wharf development. This was borne out of London's ever-increasing role as a major international financial centre during the 1980s. The Thames Barrier was completed in the 1980s to protect London against tidal surges from the North Sea.
Try to answer this question if possible (otherwise reply "unanswerable"): What event preceded a substantial decrease in the average population in London?
the Second World War