Sir Keir has been the MP for Holborn and St Pancras since 2015 and was elected Prime Minister in 2024.
Sir Keir Starmer will step down as an MP (Gareth Fuller/PA)
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He added: “It has been a huge honour and privilege to represent the best constituency in the country and to have taken over from Frank Dobson, who was such a great constituency MP and a member of the last Labour government.”
The seat of Holborn and St Pancras was created in 1983, and has only ever been held by Labour.
When will the by-election be to replace Sir Keir Starmer?
A date for the Holborn and St Pancras by-election is yet to be confirmed, but it is set to be held in 2026.
The process of triggering the by-election will begin with the Chief Whip of the Labour party, who will “move the writ”.
This entails moving a motion in the House of Commons chamber asking MPs to agree to issue a new election to replace the outgoing MP.
Andy Burnham paid tribute to Sir Keir Starmer in the House of Commons on Tuesday
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The Speaker then puts the question to MPs to decide whether to agree to the motion – if passed, this becomes an Order for the Speaker, who in turn issues a Warrant to the Clerk of the Crown who then sends the writ to the Returning Officer.
The by-election has to be held between 21 and 27 working days from the issuing of the writ.
Sir Keir will then leave the House of Commons, once a new MP has been elected to replace him.
Candidates to stand for Labour in the by-election have until Friday to apply.
Setting out the timetable for the candidate selection, the Labour Party said on its website that applications opened on Tuesday, and will close at noon on September 4.
Due diligence checks and interviews to whittle down the candidate list will then take place, before the candidate is picked after hustings and a selection meeting on September 9.