The Bow and Bromley by-election, 1940 was a parliamentary by-election held on 12 June 1940 for the British House of Commons constituency of Bow and Bromley in the Metropolitan Borough of Poplar in the East End of London. The seat became vacant when the constituency's Labour Member of Parliament (MP), George Lansbury, died on 7 May 1940, aged 81. A former Leader of the Labour Party, he had held the seat since the 1922 general election, and previously from 1910 to 1912.