Andy Burnham has won the Makerfield by-election, clearing a major hurdle for an expected challenge for the Labour leadership.
In his victory speech, he said his win could mark a turning point for the country, while he pledged not to use the constituency as a stepping stone.
The outgoing Greater Manchester mayor held off a challenge from Reform UK, which came second but more than 9,000 votes behind Labour.
In Aberdeen South, the Scottish Conservatives won a Westminster by-election for the first time in more than 50 years taking the seat from the SNP while in Arbroath and Broughty Ferry the SNP held onto the seat in its by-election overnight.
Burnham reassured voters he would not be turning away from the constituency as he headed to Westminster.
"Everyone knows that politics isn't working," he said. "Everyone can feel that the country isn't where it should be.
"Tonight could just be a turning point.
"From hereon in I will give everything I have got to make it so, to ensure the name Makerfield is for ever synonymous with bringing about the change this country needs."
He added: "It [Makerfield] will never be a stepping stone to me but instead will be my touchstone."
Makerfield has been a Labour stronghold for 120 years but the party threw everything at its campaign, following historic losses in May's elections with Labour losing ground to Reform in English council elections.
Burnham's win against his nearest challenger Robert Kenyon, a plumber and Reform councillor, came after Labour MP Josh Simons, stepped down to allow Burnham an opportunity to contest the seat.
Burnham had to be an MP if he wanted to launch a leadership challenge against the prime minister.
Speaking from the count ahead of the result, Burnham's allies stepped up calls for Sir Keir Starmer to stand aside.
Former Cabinet minister Louise Haigh, who has been managing Burnham's campaign, told the BBC she hoped the prime minister would reflect on the by-election result and results from six weeks ago.
"I hope that he will consider an orderly and managed transition."
But the prime minister has repeatedly insisted he has no intention of walking away from No 10..
If Burnham goes ahead with a challenge as expected, it would be his third leadership attempt, and former Health Secretary Wes Streeting is also expected to enter any contest.
Any change to prime minister would mean the UK seeing its seventh prime minister within a decade.
Douglas Lumsden won Aberdeen South by a convincing margin for the Scottish Conservatives, beating the SNP, with Reform UK coming third and pushing Labour into fourth place.
In his victory speech, Lumsden told his supporters: "The destruction of the North Sea oil and gas industry must stop now."
The seat had been held by the SNP's Stephen Flynn since 2019 but has shifted over the years, with a Tory MP in 2017, SNP in 2015, and Labour's Anne Begg between 1997-2015.
Lumsden is to resign as North East MSP now that he has been elected for Aberdeen South because there is a Holyrood ban on dual mandates.
The contest was held after Flynn had to give up being an MP following his election as an MSP in May.
His SNP colleague Stephen Gethins who was also elected to Holyrood in May had to give up his Arbroath and Broughty Ferry seat at Westminster.
Here the SNP held onto this seat in the by-election, with Lara Bird winning decisively by more than 5,000 votes, followed by the Tories, Reform and then Labour, who had fallen from second place in the 2024 election.
Bird said: "The people of Arbroath and Broughty Ferry have rejected the politics of division and hate that some tried to sow.
"We have demonstrated that our community is inclusive, hopeful and proud."
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