The seat became vacant when the constituency's Labour member of Parliament (MP), George Lansbury, died on 7 May 1940, aged 81.
The Labour Party selected as its candidate Charles Key, a local Alderman and leader of the Borough of Poplar.
Key was a teacher, who left the classroom when war broke out to become the deputy controller of civil defence in Poplar.
Brown had twice before been a Parliamentary candidate in Scotland (in Kilmarnock and Motherwell); she previously worked in the Ministry of Education in Moscow.
The British Union of Fascists intended to put up Mick Clarke as a candidate; this was abandoned when he became one of the first group of members rounded up under Defence Regulation 18B.