Conservative MP Nadine Dorries has resigned her seat with immediate effect – triggering a by-election contest to decide who will replace her.
The former culture secretary, 66, said it was time “for someone younger” to represent the seat.
She had already announced she would not be contesting the next general election.
“I have today informed the chief whip that I am standing down as the MP for Mid Bedfordshire, with immediate effect,” she said in a statement posted on Twitter.
“It has been an honour to serve as the MP for such a wonderful constituency but it is now time for someone younger to take the reins.”
She later edited her tweet and replaced the words “someone younger” with “another”.
Ms Dorries’ dramatic resignation comes after reports this morning that she had been dropped from Boris Johnson’s resignation honours list precisely to avoid the Conservatives facing a difficult by-election in her constituency.
Earlier this morning she had told TalkTV: “The last thing I would want to do would be to cause a by-election in my constituency.”
But interviewed again on the channel after she had announced her resignation, she said: “I can’t reveal everything – something significant did happen to change my mind”.
Seemingly confirming that she would not be given a peerage, Ms Dorries added: “There are many many people in the House of Lords who do not have the record of achievement I have but they did go to Oxford or Cambridge.”
The close Johnson ally had represented mid-Bedfordshire since 2005 and currently holds it with a large majority of nearly 25,000.
But Tory poll woes and a strong of losses in southern “Blue Wall” seats to the Liberal Democrats for Mr Sunak’s party mean opposition parties will be hoping to capture it.
Ms Dorries said she had had “a number of conversations” with ex-PM Mr Johnson over the last 24 hours while his resignation honours were being finalised, and that “he knows exactly what I am doing”.