The exact nature of the changes were as follows: 9,882 electors of the Rossendale seat were transferred to Bury North.

25,918 electors were added from the abolished Darwen constituency and 5,267 from Heywood and Royton.

1950–1983: The Boroughs of Bacup, Haslingden, and Rawtenstall, and the Urban District of Ramsbottom.

Cavendish succeeded to the peerage, becoming Duke of Devonshire and causing a by-election.

The political parties had been making preparations for an election to take place and by the Autumn of 1939, the following candidates had been selected;