The United Kingdom declared war on Germany on September 3rd 1939, two days after Germany invaded Poland. France also declared war on Germany later the same day.
The state of war was announced to the British public in an 11 AM radio broadcast by the prime minister Neville Chamberlain.
This morning, the British ambassador in Berlin handed the German government a final note stating that unless we heard from them by 11 o'clock that they were prepared at once to withdraw their troops from Poland, a state of war would exist between us. I have to tell you now that no such undertaking has been received, and that consequently this country is at war with Germany.
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