Former Conservative minister Nadine Dorries has announced she is standing down as an MP "with immediate effect".
The ex-culture secretary and close ally of Boris Johnson said it had been "an honour" to serve in Mid Bedfordshire "but it is now time for another to take the reins".
The move means there will be a by-election in her constituency, where the Tories have a comfortable 24,000 majority.
Ms Dorries had previously announced plans to stand down at the next election, and hours before her resignation, told her TalkTV channel that "the last thing I would want to do would be to cause a by-election" in her seat.
In a cryptic interview with the broadcaster later on Friday, she admitted "something significant did happen that changed my mind" but refused to be drawn on what that was.
Asked about reports the government had cut her from Mr Johnson's honours list she said: "A prime minister doesn't have the ability to change the list of a former prime minister."
Ms Dorries added that a "new life is opening up" in front of her, including a new granddaughter, and so now is the time to "do the right thing".
She said she felt a "a sense of relief" having resigned because she is unhappy with what has happened in the Tory party over the last year - namely the defenestration of Mr Johnson as PM and Rishi Sunak's appointment.
Ms Dorries has served as MP for Mid Bedfordshire since 2005 and is also a best-selling novelist.
She was promoted to culture secretary during Mr Johnson's premiership, when she lead the now-ditched plans to privatise Channel 4.
Ms Dorries, a vocal critic of Rishi Sunak's government, had been expected to be nominated for a peerage by Mr Johnson.
But she was not included on the list, which was published moments after her announcement.
According to The Times, she was scrapped from the honours list alongside former Cop26 president Alok Sharma due to fears of the damaging impact two by-elections could have on the Tories, who are flailing behind Labour in the polls.
Both MPs would have had to stand down in order to accept the peerage.
The prospects of such an electoral test would be more challenging in Sir Alok's marginal Reading West constituency, where he has a 4,000-vote majority over Labour.
The prime minister's press secretary said Mr Johnson's peerage list had been handed to the HOLAC (House of Lords Appointments Commission) unaltered "as is convention", and HOLAC then passed back its approved list.
Earlier on Friday, Ms Dorries told her TalkTV show that she did not expect to be entering the Lords "any time soon" adding: "There is a process and the last thing I would want to do would be to cause a by-election in my constituency."
Asked after resigning if she thought she should be in the House of Lords, Ms Dorries said it was "very rare" for someone from her background who was born into poverty to get a peerage.
She added that she was "slightly disappointed" but "I don't think it was to be".
Both Labour and the Lib Dems said they would be fighting to win Ms Dorries' seat.
Shabana Mahmood MP, Labour's National Campaign Coordinator, said the departure of the former cabinet minister "shows Rishi Sunak is too weak to lead a Tory Party that has lost interest in the people of Mid Bedfordshire".
"It doesn't need to be this way. Labour will be campaigning to win in this by-election by listening to the voters the Tories are ignoring."
A spokesperson for the Lib Dems said: "This is a rural seat in the heart of the blue wall, we are in it to win it."
Shortly after Ms Dorries' announcement, health minister Will Quince also revealed he won't be fighting his seat at the next election.
He said: "My decision is driven by my desire to put my family and daughters first as they grow up. I am incredibly proud of what my team and I have achieved and delivered for Colchester."
He is the 58th current MP to declare they will not contest the next election - 37 of which are serving Conservatives.