The by-election was caused by the death of the sitting Conservative MP, Sir John James Withers on 29 December 1939 at the age of 76.

The Master of Magdalene College, Allen Beville Ramsay had obtained agreement that none of the major parties would field a candidate if prominent economist, John Maynard Keynes chose to stand.

Keynes declined the invitation as he felt he would wield greater influence on events if he remained a free agent.

[2] Eventually, Keynes brother in law, 54-year-old Prof. Archibald Hill became a candidate, standing as an Independent Conservative.

Ryle was politically active at Cambridge, helping Jewish scholars emigrate from Germany and Austria before World War II.