Rishi Sunak is facing four by-elections following the resignation of another Tory MP.
David Warburton resigned with immediate effect, it emerged on Sunday, following accusations of sexual harassment and cocaine use.
In his resignation letter he said had been left with "no choice" but to provoke "the upheaval of a by-election", saying he had been denied a fair hearing by the Independent Complaints and Grievance Scheme (ICGS) and prevented from "speaking out" while it probed the accusations.
Warburton said: "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."
In an interview with the Mail on Sunday, Warburton admitted to taking cocaine after drinking "tons of incredibly potent" Japanese whiskey, but denied claims that he harassed a female political aide in his Westminster flat.
Watch: Labour fighting to win by-elections caused by Tory resignations
His resignation adds to the list of by-elections facing the prime minister this summer.
Here are the four seats up for grabs and their political history.
Somerton and Frome
Warburton's resignation triggers a by-election in the Somserset constituency of Somerton and Frome.
Warburton, who had been sitting in the Commons as an independent after having the Tory whip suspended last April over the allegations around him, took the seat from the Liberal Democrats in 2015.
He has a 19,213 majority but the Lib Dems will be hoping to reclaim the seat.
Uxbridge and South Ruislip
Boris Johnson's former seat of Uxbridge and South Ruislip is also up for grabs after the ex-PM's dramatic resignation as an MP triggered a by-election.
The by-election for the west London constituency is one of two set to take place on 20 July, alongside that of Selby and Ainsty following the resignation of Johnson's ally Nigel Adams.
The Conservatives have held the seat since 2010, and Johnson first won it in 2015.
In the 2019 election, he won 52.6% of the vote – a majority of around 7,000 ahead of Labour candidate Ali Milani.
Labour, which won 38% of the vote that year, needs a swing of 7.5 percentage points to overturn the majority.
The party's current candidate, Camden councillor Danny Beales, has said he is not taking "anything for granted" in the battle for the constituency.
Steve Tuckwell, Hillingdon Borough councillor and deputy chairman (political) of Uxbridge & South Ruislip Conservative Association, has been selected as the Conservative candidate for the seat.
Selby and Ainsty
A second by-election on 20 July will take place for the Selby and Ainsty constituency in North Yorkshire.
Nigel Adams quit as a Conservative MP after being denied a peerage in former PM Johnson's resignation honours list.
He had held the seat since it was established in 2010, winning with a majority of 20,137 in 2019, building on his 2017 majority of 13,772, with Labour coming second on both occasions.
The Conservatives were due to be represented by Michael Naughton, but he has since withdrawn from the contest due to "an unforeseen family medical emergency".
Labour has announced 25-year-old Keir Mather as its candidate.
If elected, Mather would be the youngest MP in the Commons, as the current youngest – Labour’s Nottingham East MP Nadia Whittome – is 26.
Mather said: "It is an honour to have been selected to fight to represent this brilliant constituency.
"Whilst the Conservatives are too distracted by their own chaos to deliver, the people of Selby and Ainsty have been left without a voice.
"We are facing the worst fall in living standards in a generation. Mortgages are up, our public services are broken, and the economy is stagnating.
"It's time for change."
Former Cabinet minister and MP for Mid-Bedfordshire Nadine Dorries announced she was standing down with "immediate effect" on 9 June via Twitter.
But despite announcing her resignation, it has been reported that Dorries is staying on as an MP but plans to stand down at a later date – creating the potential for an autumn by-election as MPs are due to go on their summer recess after 21 July.
Mid-Bedfordshire has been a Conservative seat since 1931 and has been held by Dorries since 2005.
She had a majority of 24,664 at the 2019 general election and both Labour and the Liberal Democrats will be hoping they can win the seat.
Labour has named Alistair Strathern, a former teacher who works for the Bank of England and is a cabinet member on Waltham Forest Council in east London, as its candidate.
Despite the lack of formal resignation from Dorries, the Conservatives have named its candidate as Festus Akinbusoye, who has been the Bedfordshire police and crime commissioner since 2021.
Watch: Nadine Dorries insists it's 'still my intention to resign'
Liberal Democrat deputy leader Daisy Cooper MP said: "This by-election is going to be a close two-horse race between the Liberal Democrats and an out-of-touch Conservative party.
"We have already heard from lifelong Conservative and Labour voters across Bedfordshire who are backing the Liberal Democrats to send this government a message.
"People in Mid-Bedfordshire feel let down and taken for granted by this Conservative government.
"GP appointments in Bedfordshire are hard to come by, whilst NHS dentists are impossible to find. Local NHS services have been badly neglected.
"Next month, Mid-Bedfordshire has the chance to send this chaotic sham of a government a message."
Mid-Bedfordshire often attracts a range of candidates, with the Monster Raving Loony Party standing in the previous three elections.
Other candidates announced for the seat include Gareth Mackey, an independent councillor on Central Beds Council, Cade Sibley for the Green Party, Dave Holland for Reform UK and Alan Victor for the True and Fair Party.