The by-election to replace Nadine Dorries as the MP for Mid Bedfordshire is due take place in mid-October, after the government formally triggered the contest.
It means Rishi Sunak faces another embarrassing defeat ahead of next year’s general election.
Dorries dramatically announced in June she was quitting parliament after being denied a seat in the House of Lords.
But she delayed her formal resignation until the end of August, preventing a by-election from taking place before the summer.
On Monday the government moved the writ - the formal process needed to hold a by-election - dated September 12.
According to the BBC the contest is likely to take place on October 12 or October 19.
It means Sunak would avoid an election defeat before the Conservative Party’s annual conference which runs from October 1 until October 4.
Dorries holds a healthy majority of 24,664 in Mid Bedfordshire, a lead that would usually be considered ultra-safe.
But both Labour and the Lib Dems have overturned similarly large Tory majorities over the course of this parliament.
The government moved the writ after the Lib Dems threatened to do it themselves in order to force an ear by-election.
It is usually the party whose MP has resigned that decides the timing of electing the replacement, but there is nothing to stop another party from attempting it.
Dorries has launched several attacks on Sunak since she announced she was stepping down, including warning the Tories will lose the next election with him as leader,
The former culture secretary also suggested she might not even vote Conservative.