The British Prime Minister during World War II was Winston Churchill. He served as Prime Minister from 1940 to 1945, during the most critical years of the conflict. Churchill is renowned for his leadership during the war, particularly for his stirring speeches that rallied British morale during dark times. His commitment to defeating Nazi Germany and his role in forging alliances with the United States and other nations were pivotal in the eventual Allied victory. After the war, he continued to influence British and global politics, notably contributing to post-war discussions on rebuilding Europe.