Andy Burnham has won the crucial Makerfield byelection by a huge majority, paving the way for a challenge to Keir Starmer’s premiership.

The Greater Manchester mayor beat the Reform UK candidate, Robert Kenyon, and the new hardline Restore Britain party.

Labour won 54% of the vote to Reform UK’s 35%, while Restore Britain secured 7%. Turnout was 58.75% – six points up on the general election, with 45,510 votes cast.

After the most consequential byelection in modern British history, Burnham is widely expected to mount a bid for No 10 if a formal leadership contest is triggered in the coming days.

However, his allies believe Starmer should be given time to set out a timetable for his departure, the Guardian revealed on Thursday, with some in his team having talked ministers out of resigning as soon as this weekend to stop the government falling into chaos.

The “King of the North”, who was first elected to parliament almost exactly 25 years ago and served in the governments of Tony Blair and Gordon Brown, will return to Westminster nine years after he left.

His win means Labour faces another gruelling contest against Reform UK, this time to retain the Greater Manchester mayoralty. The byelection of 2 million voters will be one of the biggest in British political history, and is expected to be held on 30 July.

The Makerfield byelection was triggered when Josh Simons, the then Labour MP, agreed to stand down last month to allow Burnham to contest the seat and mount a challenge to Starmer’s ailing leadership.

The prime minister has faced calls for his resignation over the disastrous appointment of Peter Mandelson as US ambassador, followed by elections in which Labour lost more than 1,200 local councillors and control of the Welsh Senedd in May.

Hours before voting opened, Burnham said the outcome of the byelection would change British politics for years to come.

“This byelection is going to put power where it should be, in the hands of people here, in the hands of you, in the hands of 75,000 people across this constituency, a place that has been overlooked by Westminster in the past,” he told supporters at a sports centre in the constituency.

“It’s right that this place decides what happens next, the change that comes to British politics, because change is coming, but the question tonight is what kind of change.”

Burnham said his offer would “make life more affordable to people again, to give the British people a bit more money in their pockets, to give people a bit more breathing space in their daily lives”. He said the result would be a “vote to power up the north of England”.

Starmer has said he will not walk away from his post and that he intends to fight any challenge.

The prime minister is also facing a looming threat from his former health secretary Wes Streeting, who has indicated he is prepared to trigger a Labour leadership contest as early as next week.