Labour has comfortably won the Chester by-election, the first public vote since Boris Johnson and Liz Truss were forced out of Number 10.
Samantha Dixon retained the seat for her party with 17,309 votes, a 61% share and nearly 11,000 more than the Conservative candidate.
Labour was expected to win, but the margin was bigger than in 2019 when the gap was 6,164 votes.
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It represents a 13.8% swing from Tory to Labour, the sixth biggest between the parties since 1945, and more than the 12.7% in Wakefield in June.
It is also the worst result for the Conservatives in Chester since 1832, with candidate Liz Wardlaw getting 6,335 votes or 22.4%.
Ms Dixon told Sky News the result was a “resounding mandate” for Labour, and in her victory speech said the Tories were “on borrowed time and people want to change”.
“People in Chester and across our country are really worried,” she said.
“Worried about losing their homes because they can’t afford the mortgage repayments or the rent, worried about whether they can put the heating on, worried about whether they can put food on the table for their families.
“This is the cost of 12 years of Conservative government. The government, which has wreaked havoc with our economy, destroyed our public services and betrayed the people who put their trust in them at the last general election.”
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Jon Craig, Sky’s chief political correspondent, described it as a “crushing defeat” for the Tories and “ominous” for the party’s chances at the next election.
However, he said it would be harsh to personally blame the prime minister for the result.
This is the third consecutive disappointment for the Tories in by-elections after Labour snatched Wakefield and the Lib Dems secure a historic victory in Tiverton and Honiton over the summer.
The Chester contest was triggered by the resignation of Christian Matheson, the Labour MP who was suspended in October after two allegations of sexual misconduct from a former staff member were upheld.
Shadow international trade secretary Nick Thomas-Symonds said it was “a landslide result” that “points to a majority Labour government” at the next election.
But he told Sky News his party was not “complacent”, adding: “We know the job there is to do to continue winning the trust of the British people. That is what we are going to continue to do right up to the day of the next general election.”
The result also follows a slump in opinion polls for the Conservatives after a raft of reshuffles in their top ranks.
The party had a small bounce in their numbers after Rishi Sunak entered Downing Street last month – the third Tory leader this year – but they are still regularly polling behind Sir Keir Starmer’s Labour.
Tory peer and pollster Lord Hayward said the Conservatives would be “relieved” they still managed to get over 20% of the vote, as the result was “not quite as bad as the opinion polls had been suggesting”.
He told Sky News it was now down to Mr Sunak to “get on with the job” ahead of the next big electoral hurdle – the local elections next May.
“The [Tories] have a real challenge on their hands,” he said. “But Rishi comes across to the public at large as managerial, his ratings are way ahead of the Tory Party… so he is showing there is potential.
“But the cost of living and strikes are clearly big issues. Rishi has to convince the public he can manage out of this crisis.”
This was the first of two by-elections this month, with the second on 15 December in Stretford and Urmston, Greater Manchester.
That vote was called after Labour’s former shadow education secretary, Kate Green, announced she was standing down to become deputy mayor of Manchester.