The Declaration of war by France and the United Kingdom was given on 3 September 1939, after German forces invaded Poland. Despite the speech being the official announcement of both France and the United Kingdom, the speech was given by the British Prime Minister Neville Chamberlain, in Westminster, London.

Based on this passage, britain and france declared war on germany when? Let's think fast. Stream of consciousness:
To answer the question, consider the following: The Declaration of war by France and the United Kingdom was given on 3 September 1939, after German forces invaded Poland. So the final answer is 3 September 1939.