The Tories held onto the seat but the huge 10 percent swing to Labour was well above the six percent recorded on average by national opinion polls, and also the biggest swing in a by-election for Sir Keir Starmer's party in seven years. Polling expert Sir John Curtice said the swing was more down to a 13 point fall in support for the Tories than the seven point increase in Labour's tally - Sir Keir's party had improved by more in nearly a dozen by-elections over the past decade, but still lost the next general election.

       The polling expert warned the biggest concern for the Prime Minister is the wins gained by the Reform Party, the successor to the Brexit Party, which won seven per cent of the vote in the by-election.

       Sir John explained in an article for The Times: "The foundation of Boris Johnson’s victory in 2019 was uniting most of the Leave vote behind him, a success that was assisted by Nigel Farage’s decision not to contest Conservative-held seats.

       "In contrast the Remain vote was fragmented between Labour, the Liberal Democrats and the nationalist parties. The Conservatives cannot afford for the Leave vote to become fragmented.

       "Small parties such as Reform UK are better able to enjoy success in by-elections than in a general election. The party is also relatively unknown to many voters. Johnson has to hope it remains that way."

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       Additional reporting by Bill McLoughlin