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]”.

Mr Warburton’s decision to quit means the Conservatives face four by-elections at a time when they are trailing behind Labour in the national polls.

All eyes are on Mr Johnson’s marginal west London constituency, where the Conservatives won a 53% share of the vote at the last general election in 2019.

Mr Warburton and Mr Adams are cushioned by larger majorities of 19,213 and 20,137 respectively, but Labour and the Liberal Democrats are already out campaigning to try and swing the constituencies in their favour.

The by-elections come at a difficult time for the prime minister, who is struggling to draw a line under the Boris Johnson partygate and honours row, while also facing pressure to help families with soaring mortgage rates.

Mr Warburton has admitted the government faces a “tough” contest in his seat, which was held by the Lib Dems before he entered parliament in 2015.

“It’s going to be tough for them. I mean, I’ve won three large majorities in a row, but prior to that it was a Lib Dem seat for 18 years over four elections,” he told GB News.

“It’s going to be difficult and the government is obviously facing three other by-elections … it’s a tough time for by-elections.”

Ms Dorries also has a large majority of 24,000 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.

Mr Johnson had sought to elevate the former culture secretary to the House of Lords along with Mr Adams but when the list was finally revealed, their names were absent – prompting a bitter war of words between the former prime minister and Mr Sunak.

Ms Dorries announced her intention to resign just before the list was published, with Mr Johnson following her hours later over the report from the privileges committee, and Mr Adams quit the next day.

Mr Johnson was found to have lied to MPs over the Downing Street parties scandal and was stripped of his special access to parliament after the House of Commons voted by 354 to seven to back the inquiry’s findings.