Between 7 September 1940 and 21 May 1941, 16 British cities suffered aerial raids with at least 100 long tons of high explosives. Over a period of 267 days, London was attacked 71 times, Birmingham, Liverpool and Plymouth eight times, Bristol six, Glasgow five, Southampton four, Portsmouth and Hull three and a minimum of one large raid on eight other cities. This was a result of a rapid escalation starting on 24 August 1940, when night bombers aiming for RAF airfields drifted off course and accidentally destroyed several London homes, killing civilians, combined with the UK Prime Minister Winston Churchill's retaliatory bombing of Berlin on the following night.[clarification needed]