Liz Truss is set for her first by-election as prime minister after a Labour MP announced she was quitting parliament.
Rosie Cooper said she was resigning as MP for West Lancashire to become the chair of Mersey Care NHS foundation trust.
In 2017, British neo-Nazi Jack Renshaw launched a plot to murder Cooper and was jailed for life in May 2019.
Cooper - who held the safe Labour seat at the last election with a majority of 8,336 - said her decision had come 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.
“This appointment means I am unable to continue with my other passion of representing the people of West Lancashire as their MP, being their voice and fighting their corner as I have done since 2005.”
In a statement on Twitter, Cooper said it had been an “incredible honour and privilege to have served the people of West Lancashire for the last 17 years”, adding: “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 re-selection to stand as West Lancashire’s Labour Party candidate for the next general election,” she added. “This was prior to the recruitment process for the Mersey Care position.
“The 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.”
Cooper, who has previously worked in various roles at the NHS and has served on parliament’s health and social care committee, was first election an MP in 2005.
She said her return to the health service “felt like the right opportunity at the right time”.
Cooper added: “There is no question I am very sad that my time in West Lancashire has come to an end. I will leave certain that I gave everything I had to this role which I love and to people who have come to mean a great deal to me.
“To the residents of West Lancashire, I wish you the very best for the future and I will continue to take an interest from a distance.
“You have given me more than I can ever explain, and I want to thank you for the faith you placed in me to be able to serve you for 17 years.”