Sir Keir Starmer has declared “Labour is back” after the party held on at the Batley and Spen by-election.
The Labour leader headed to the West Yorkshire constituency to celebrate Kim Leadbeater’s narrow 323-vote win.
He said it was a victory for “hope over division, and decency over hatred”, after a bitter campaign, and “just the start” for Labour.
Ms Leadbeater will now represent the seat previously held by her sister Jo Cox, who was murdered there in 2016.
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Sir Keir hailed her “incredible courage” in standing for the seat, and claimed “Labour is coming home”.
The result will come as a relief to Sir Keir, who has been under pressure following recent election defeats for his party.
Prime Minister Boris Johnson said the Conservative candidate Ryan Stephenson ran an “incredibly positive campaign” and “did very well to reduce a longstanding Labour majority”.
Speaking at a Downing Street press conference with German Chancellor Angela Merkel, Mr Johnson claimed the by-election result was the “third biggest swing in favour of the government since the War”.
Ms Leadbeater took the seat with 13,296 votes, giving her a slim victory over Mr Stephenson with 12,973.
Former Labour and Respect MP George Galloway, standing for the Workers Party of Britain party, came third with 8,264.
The former Bradford West MP had sought to win support from the seat’s Labour voters, with the aim of toppling Sir Keir as Labour leader.
His campaign had targeted voters unhappy at the party’s stance on issues including the conflict between Israel and the Palestinians, and the disputed territory of Kashmir, claimed by both India and Pakistan.
An ethnically diverse seat, about 20% of the population is from an Asian background, with just under 19% being Muslim.
Speaking in Cleckheaton, Sir Keir said Ms Leadbeater had won despite a split in Labour’s vote, because “former Tory voters voted for her”.
“The Tories had a clean hit, nobody was going against them, and they didn’t win,” he added.
Sir Keir was reportedly facing a potential challenge from his deputy Angela Rayner if the party lost another northern English constituency to the Tories.
It comes after the Tories took Hartlepool, a once rock-solid Labour seat in May, alongside losing more than 300 councillors in England’s local elections.
But Andrew Scattergood, co-chair of the Momentum campaign set up to campaign for former Labour leader Jeremy Corbyn, said the party could not “portray this as a massive victory”.
“We squeaked it over the line and there are some serious concerns,” he told the BBC News Channel.