Keir Mather won the North Yorkshire seat as support for the Conservatives there crumbled.

He also became the youngest MP in Parliament at 25 years old on a night of three by-elections.

“This is a historic result that shows that people are looking at Labour and seeing a changed party that is focused entirely on the priorities of working people with an ambitious, practical plan to deliver,” Sir Keir said.

Read more: Sunak avoids by-election wipeout as Tories hold Uxbridge but face double defeat to Labour and Lib Dems

“Keir Mather will be a fantastic MP who will deliver the fresh start Selby and Ainsty deserves.

“It is clear just how powerful the demand for change is. Voters put their trust in us – many for the first time.

“After 13 years of Tory chaos, only Labour can give the country its hope, its optimism and its future back.”

He won it with a majority of 4,161, overturning the Conservatives’ 20,137 majority in the 2019 election.

That is Labour’s biggest overhaul of a majority in a by-election in its history.

Mather beat Claire Holmes, the Conservative candidate, who had hoped to retain the seat for her party after Tory MP Nigel Adams quit.

The Boris Johnson supporter resigned after he was denied a peerage.

Read more: Tories maintain hold on Boris Johnson’s former Uxbridge seat in by-election blow to Labour’s Keir Starmer

Mather said of his win in a Tory stronghold: “I want to begin my time as your MP by being very clear: I understand the enormity of what has just happened, I know what an honour this is, and I am humbled by this opportunity to serve.

“We have re-written the rules on where Labour can win. Thousands of the votes cast yesterday were in Labour’s box for the very first time. For those of you who have put your faith in our party: thank you.”

The blow to Rishi Sunak was softened by the Tories just managing to hold on to Boris Johnson’s former seat of Uxbridge.

Steve Tuckwell won by 495 votes in what the Conservatives turned into a referendum on the controversial Ulez charge in London.

The Liberal Democrats also managed to win Somerton and Frome with Sarah Dyke.

Liberal Democrat Leader Sir Ed Davey said the “stunning victory” for his party showed that it was “firmly back” in the West Country.