Rosie Cooper has formally stood down as Labour MP for West Lancashire sparking a Red Wall by-election.
She was previously the target of a murder plot and admitted "events I have faced have taken their toll".
Far-right extremist Jack Renshaw was handed a life sentence for plotting to murder the MP in 2019.
His attempt was foiled by whistleblower Robbie Mullen, who was at a meeting in a pub when Renshaw announced he was going to kill Ms Cooper in July 2017.
Ms Cooper has been appointed Steward and Bailiff of the Three Hundreds of Chiltern by the Chancellor, the archaic process that effectively resigns an MP.
She previously said she had accepted the position of chair of Mersey Care NHS Foundation Trust "after a considerable period of soul-searching and reflection".
This appointment meant she was unable to continue with her duties as an MP, she said.
Ms Cooper, who has held the seat for 17 years, has a current majority of just over 8,300.
Her resignation will mean a by-election in a constituency firmly in the "Red Wall" which Sir Keir Starmer will be looking to hold.
A statement from the Treasury said: "The Chancellor of the Exchequer has this day appointed Rosemary Elizabeth Cooper to be Steward and Bailiff of the Three Hundreds of Chiltern."
In a statement posted to Twitter by Ms Cooper in September, she said: "It has been an incredible honour and privilege to have served the people of West Lancashire for the last 17 years. I have loved every minute, even in the most difficult times.
"I appreciate this will come as a surprise to many people having recently secured reselection to stand as West Lancashire's Labour Party Candidate for the next general election.
"This was prior to the recruitment process for the Mersey Care position.
"This decision to apply for the role was taken after a considerable period of soul searching and reflection.
"The events I have faced over the last few years are well documented and undoubtedly have taken their toll."
The by-election could offer a return to Westminster for Greater Manchester Mayor Andy Burnham, who has stoked speculation in recent weeks that he still harbours ambitions of being Labour leader.
However, he has insisted he would serve his full second term as mayor.
