Labour's Samantha Dixon has increased her majority in the Chester by-election to an impressive 61 percent, receiving 17,309 votes. This is nearly 11,000 more than the Conservative candidate Liz Wardlaw, who got 6,335 votes or 22.4 percent of the share.
The election in Chester was triggered after incumbent MP Chris Matheson resigned following allegations of sexual misconduct.
The Conservatives have experienced two by-election defeats over the summer, as Labour won power in Wakefield and the Lib Dems made history with their victory in Tiverton and Honiton.
Mr Sunak's party is still experiencing a slump in the polls after months of turbulence due to Boris Johnson stepping down, the short-lived premiership of Liz Truss and the economic downturn following Kwasi Kwarteng's mini-budget.
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Despite an upwards incline in numbers when Mr Sunak became PM, the Tories are still polling behind Labour.
Another by-election is set to take place on December 15, with Stretford and Urmston in Greater Manchester up for grabs.
The Manchester vote was called after MP Kate Green, Labour's former shadow education secretary, said she was stepping down to become the city's deputy Mayor.
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Speaking after the results were announced, Ms Dixon said people in Chester "have said unreservedly that Rishi Sunak's Conservatives no longer have a mandate to govern".
She continued: "I don't think they believe that the Conservatives have the answers, I think they think it's Labour's turn now."
Election officials earlier confirmed that turnout was 41.2 percent, with a total of 28,541 votes cast overall.
Labour were predicted to win the election, however, this vote represents a 13.8 percent swing from the Tories to Sir Keir Starmer's party, more than the 12.7 percent swing at Wakefield in June.
It is also the sixth largest swing between the parties at a by-election since 1945 and the worst results for the Tories in Chester since 1832.
Ms Dixon told Sky News the result was a "resounding mandate" for Labour, and in her victory speech warned the Tories that they were "on borrowed time and people want to change".
Sky's chief political correspondent Jon Craig said it was a "crushing defeat" for Mr Sunak's party and "ominous" for the next general election, however, he added it would be unfair to solely blame the PM for the result.
