Labour's former senior whip Darren Hughes has written to Parliament's Speaker resigning as an MP.
Mr Hughes faces a police investigation into a complaint against him of sexual assault.
The resignation is effective from Tuesday, the day Parliament resumes sitting after a one-week break.
At that time the Speaker, Lockwood Smith, will announce Mr Hughes has resigned and that there is now a vacancy.
That vacancy could be filled by former minister Judith Tizard, who is the next on Labour's party list.
But the party is trying to convince her and four others to decline so that Louisa Wall succeeds Mr Hughes in Parliament.