The seat became vacant as a result of the death of Sir Rhys Hopkin Morris on 22 November 1956.

Hopkin Morris's majorities had been narrow and it was recognised that a significant number of voters had favoured him personally and might not transfer their allegiance to a new Liberal candidate.

Previously selected prospective candidate for Labour, Brynley Thomas, stood down in her favour.

The local Liberals created another dilemma by choosing John Morgan Davies as their candidate; the Liberals were seeking to challenge the ruling Conservatives over the ongoing Suez Crisis and Morgan Davies, like Hopkin Morris, had publicly taken a pro-government stand on the issue.

Jo Grimond, who had assumed leadership of the Liberal Party only a few weeks before, had to choose whether to support or disown Morgan Davies.