The Labour MP Rosie Cooper is expected to stand down and trigger a byelection in her West Lancashire constituency this autumn, after taking on a new role with the NHS. Cooper, who was targeted as part of a murder plot by members of a banned neo-Nazi group, said she had undergone a “considerable period of soul-searching” and admitted the events had “taken their toll”. She said it had been “an incredible honour and privilege” to serve in the Commons since 2005, and acknowledged her departure would “come as a surprise to many people” given she had recently been reselected to stand at the next general election. While her statement did not confirm when she would step down from parliament, the Guardian understands she is expected do so by the end of November, meaning a byelection will be called in the north-west England seat that she held with an 8,000 majority in 2019. Cooper said she was “very sad that my time in West Lancashire has come to an end” and added that she would become chair of Mersey Care NHS foundation trust. Labour sources are hopeful the party will win the byelection comfortably, given its performance at the local elections in May and its retaking of a “red wall” seat – Wakefield – the following month. The contest will still be viewed as a test of Liz Truss’s ability as prime minister to turn the Conservatives’ fortunes around, having been languishing in the polls since last December. “Team Truss will want to avoid a collapse in vote share in their first electoral test,” said Rob Ford, an elections expert. West Lancashire has been Labour since 1992, before which it was held with a relatively small majority by the Conservatives. Cooper has remained a backbencher throughout her time in parliament, and is a long-serving member of the health and social care select committee. She spoke in the Commons several years ago about being the intended victim of an “act of terrorism”, telling fellow MPs: “I was to be murdered to send a message to the state, and to send a message to this place.” Cooper said in April 2019: “Members of this house are regularly abused and attacked. Our freedoms, our way of life, our democracy is under threat, and we must do our utmost to defend it.”