The first by-election of 2023 will take place in February after a Labour MP quit for a role in the NHS.
A writ has been moved – meaning the correct paperwork has been filed – for the West Lancashire by-election.
The seat is up for grabs after Rosie Cooper formally resigned as an MP in November to become chairwoman of the Mersey Care NHS Foundation Trust.
Ms Cooper, 72, has held the Labour stronghold since 2005, securing more than 52% of the vote and a majority of 8,336 at the last election.
Having been created in 1983, the seat was held by Tory MP Ken Hind between then and 1992, when Labour’s Colin Pickthall won it. Labour has had control of the constituency ever since.
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.
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Ms Cooper, who was originally a Liberal Democrat, was previously chair of Liverpool Women’s Hospital and a trustee of the Roy Castle Lung Cancer Foundation.
She said returning to the NHS “felt like the right opportunity at the right time”, but means she cannot continue “with my other passion of representing the people of West Lancashire as their MP”.
The by-election has to take place between 21 and 27 working days from the issuing of the writ, suggesting it will take place on 9 February.
It will be Prime Minister Rishi Sunak’s first big test at the ballot box this year, and comes as the government grapples with a continuing wave of industrial action.
Labour have been buoyed by two recent by-election victories.
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Last month, the party comfortably held on to its seat in Stretford and Urmston, Greater Manchester, with a 10.5% swing from Conservatives to Labour.
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The by-election was triggered after Labour former minister Kate Green resigned in November to become Greater Manchester deputy mayor.
The victory came just two weeks after a successful result in Chester, when Samantha Dixon held the seat for Labour with an increased majority of some 11,000 over the Tories.