Rosie Cooper has stood down as Labour MP for West Lancashire.
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.
It comes two months after she announced she had accepted the position of chair of Mersey Care NHS Foundation Trust following “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 was the target of a murder plot by a neo-Nazi in 2017, said the events she had faced over recent years had “taken their toll”.
Jack Renshaw, of Skelmersdale, Lancashire, is currently serving a life sentence for planning to kill her.
The MP for West Lancashire, 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 the constituency, which sits firmly in the “Red Wall”.
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.”
The Steward and Bailiff of the Three Hundreds of Chiltern is a historical position, which in modern times has been used to facilitate the resignations of MPs.
Elected MPs are unable to resign and must become disqualified if they wish to leave the Commons before the end of the Parliament.
Holding the position immediately disqualifies a person by law from being an MP and therefore removes them from the House of Commons.
The by-election for West Lancashire will be triggered by the Labour chief whip issuing a writ for the vacant seat.
The vote has to take place between 21 and 27 working days from this point.
