Thursday 24th June 2021
- Bill Brisbane (Liberal Democrat) – elected for Chichester East (Chichester) due to the resignation of Kevin Hughes (Labour) [Liberal Democrat gain from Labour]
- Barrie Hall (Liberal Democrat) – elected for North Curry and Ruishton (Somerset West and Taunton) due to the resignation of Philip Stone (Liberal Democrat)
- Becky Maoudis (Conservative) – elected for Wolvey and Shilton (Rugby) due to the resignation of Chris Pacey-Day (Conservative)
- Robert McKendrick Jnr (Independent) – elected for Murdostoun (North Lanarkshire) due to the death of Robert McKendrick Snr (Independent)
- Gwynfor Owen (Plaid Cymru) – elected for Harlech (Gwynedd) due to the resignation of Freya Bentham (Independent) [Plaid Cymru gain from Independent]
- Kate Tomlinson (Conservative) – elected for Priory Vale (Swindon) due to the resignation of Emma Faramarzi (Independent, was Conservative)
Thursday 17th June 2021
- James Bartlett (Conservative) – elected for Upper Culm (Mid Devon) due to the death of Glanmor Hughes (Conservative)
- Sarah Green (Liberal Democrat) – elected for Chesham and Amersham (House of Commons) due to the death of Cheryl Gillan (Conservative) [Liberal Democrat gain from Conservative]
- Steve Griffiths (Liberal Democrat) – elected for Old Cleeve and District (Somerset West and Taunton) due to the resignation of Peter Pilkington (Liberal Democrat)
- David Keating (Conservative) – elected for East Garioch (Aberdeenshire) due to the death of Fergus Hood (Liberal Democrat) [Conservative gain from Liberal Democrat]
- Julie Brociek-Coulton (Labour) – elected for Sewell (Norfolk) in postponed poll from May elections
- Susan Carey (Conservative) – elected for Elham Valley (Kent) in postponed poll from May elections
- Gary Champion (Green) – elected for Sewell (Norwich) in postponed poll from May elections
- Judy Moore (Independent) – elected for Felbridge (Tandridge) in postponed poll from May elections
Thursday 10th June 2021
- Uzma Rasool (Labour) – elected for Grove Green (Waltham Forest) due to the death of Chris Robbins (Labour)
- Jennifer Whilby (Labour) – elected for Lea Bridge (Waltham Forest) due to the resignation of Yemi Osho (Labour)