The 1940 Bow and Bromley by-election, 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 leader of the Labour Party, he had held the seat since the 1922 general election, and previously from 1910 to 1912.