Less than two years after Starmer won a landslide election victory that promised to end Britain's chaotic politics, the prime minister had spent the weekend considering whether to step aside or fight a leadership contest.
Starmer announced his decision on Monday, stepping out of Downing Street accompanied by his wife Victoria to cheers and applause from his team.
"Walking up this street two years ago was the proudest moment of my life," he said, announcing he would resign as leader of the Labour Party and do everything he could for "an orderly handover".
He said he had advised King Charles of his decision and would remain as prime minister until a successor was chosen.
Skills Minister Jacqui Smith told Times Radio early on Monday she would have liked Starmer to stay on but he had been weighing what was best for the country, due to the "pressure that is being brought upon him".
The threat to Starmer, which had been building for months, increased sharply on Friday when Burnham, the Greater Manchester mayor, decisively won a parliamentary election to return to Westminster, beating a candidate from Nigel Farage's Reform UK party, which has led national opinion polls for more than a year.
That victory gave hope to Labour MPs that Burnham, a career politician known for his communication skills, could transform the fortunes of a party that has lost support under Starmer, whose popularity ratings had sunk to the lowest for any British leader.
But the widely expected change of leader is not without risk.
Beyond saying that the country needs fundamental change and to bring down the cost of living, Burnham has yet to make clear his approach to foreign affairs, the economy and defence.
Like Starmer, he could find he has little room to manoeuvre, hemmed in by bond market investors opposed to any additional borrowing, and confronted by an angry electorate who believe the country is not working properly.
Britain already has the highest borrowing costs in the Group of Seven wealthy nations due to its high debt and interest payments, years of anaemic economic growth, its struggles to cut spending and the need to invest in areas such as defence.
Investors spoken to by Reuters were divided over whether Burnham, who said last September that Britain had to get "beyond this thing of being in hock to the bond markets", would respect the need to reassure markets.
He has since said he was misrepresented.
"In our view, a Burnham premiership would inherit a precarious fiscal situation with few tools to deliver meaningful change," economists at Citibank said on Friday.
Starmer had said on Friday he would stand in any formal Labour leadership contest that sought to replace him. But that appeared to change at the weekend.
Any change in leader, without even a contest within the party, could anger voters who will not have had a say in who is running the country from Downing Street.
The former health minister Wes Streeting had said he had the backing of the 81 Labour MPs needed to enter a leadership race, but one senior figure in the party said they believed Streeting could do a deal with Burnham, giving him a senior role if he stayed out of the contest.
While Starmer's team believes his landslide national election win in 2024 gave him the mandate to stay in post until 2029, business minister Peter Kyle said on Sunday the prime minister had reflected on "the political challenges that he faces in this moment".
Burnham, if he succeeds, would become Britain's seventh prime minister since the Brexit vote to leave the European Union, which took place 10 years ago this week.
