Who was the British prime minister at beginning of World War 2?

Neville Chamberlain- he declared war on Germany on 3rd September 1939 after Hitler invaded Poland, thereby breaking the Munich Declaration which Chamberlain believed he had secured the previous year.

He went on to send a vast British expeditionary Force of 150,000 men and armaments to France, to support the Frence in the battle to stop the Nazis invading France. But although the combined Anglo-French Armies inflicted heavy losses on the Germans, they could not prevent the advance of Hitler's Panzer tank divisions from driving them back towards the Dunkirk beach head. A huge emergency evacuation campaign ensued, after which Chamberlain was discredited as a competent wartime leader and resigned in May 1940.

He was replaced by Winston Churchill, who took office as head of a Coalition Government only about a fortnight before the Battle of Britain began.