Who was the last British prime minister to die in office?

The last British prime minister to die in office was Sir Winston Churchill, who served from 1940 to 1945 and again from 1951 to 1955. However, he did not die while actively in office; the last prime minister who did was Lord Palmerston, who passed away in 1865 while serving as prime minister. Churchill resigned in 1955 and died in 1965, after his political career had ended.