Rishi Sunak is facing a by-election triple threat after the Somerton and Frome vote was scheduled for 20 July.
The announcement comes after David Warburton formally left the House of Commons on Monday.
The election will take place on the same day as Boris Johnson’s vacated Uxbridge and South Ruislip seat, and Nigel Adams’ Selby and Ainsty seat.
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There is still no answer on when Nadine Dorries will leave her Mid Bedfordshire constituency, after saying she was “standing down with immediate effect” almost two weeks ago.
Mr Warburton announced he was standing down as an MP last week, having been suspended from the Conservative Party since April 2022 while being investigated by the Independent Complaints and Grievance Scheme (ICGS).
He had been accused of sexual harassment and drug abuse.
In his resignation letter, Mr Warburton said the “malicious allegations” and “flawed” 14-month investigation had “inflicted unimaginable and intolerable destruction on my family and on [himself]”.
He said had been left with “no choice” but to provoke “the upheaval of a by-election”, adding: “It is my hope that, in so doing, I can freely illuminate the methods of an oversight system not fit for purpose, so that friends and colleagues in the House can see the perverted process by which their own judgement may at any time be freighted.”
Mr Warburton’s decision to quit means the prime minister faces three potentially difficult by-elections in one day,
All eyes are particularly on Mr Johnson’s marginal west London constituency, where the Tories had a 53% share of the vote at the last general election in 2019.
Mr Warbuton and Mr Adams are cushioned by larger majorities of around 20,000, but there is concern among Tories as the party continues to trail behind Labour in the national polls.
Ms Dorries similarly has a large majority, but she is holding off the formal process of resigning while she seeks answers from Downing Street about why she did not get her peerage.