2023 Selby and Ainsty by-election
20 July 2023
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Selby and Ainsty constituency | |||||||||||||||||||||||||||||
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| Registered | 80,150 | ||||||||||||||||||||||||||||
| Turnout | 44.8% ( 27.2 pp) | ||||||||||||||||||||||||||||
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A by-election for the United Kingdom parliamentary constituency of Selby and Ainsty was held on 20 July 2023, following resignation of incumbent Conservative Party member of Parliament (MP) Nigel Adams. Keir Mather of the Labour Party won the seat, setting a record for the largest majority ever overturned by the party in a by-election at the time.
The 2023 Somerton and Frome by-election and 2023 Uxbridge and South Ruislip by-election were held on the same day; the Labour candidate in the later failed to win in the seat despite it being much more marginal than Selby and Ainsty.
At the time, this was the second-largest swing to Labour in a by-election since 1945—behind only Dudley West in 1994—but it was then exceeded in the 2023 Tamworth by-election held in the following October and now stands as the third-largest; the 2023 Mid Bedfordshire by-election held the same day would also see Labour overturn a larger majority than in Selby.