Under pressure over the appointment of a man whose close ties to the late US sex offender Jeffrey Epstein have come into full focus, Starmer has attempted to change the narrative.
Starmer has told Labour MPs he had "won every fight I've ever been in" as he vowed not to "walk away" amid calls for him to resign.
The prime minister addressed a packed meeting of the Parliamentary Labour Party in Westminster on Monday in the wake of Scottish Labour leader Anas Sarwar's call for him to step down.
The new scandal over Mandelson, who was sacked as ambassador to the US in September, came after files released by the US Justice Department last month included emails suggesting Mandelson had leaked discussions on possible UK asset sales and tax changes to Epstein during the financial crash.
The resignation of communications chief Tim Allan followed the exit of Starmer's closest aide Morgan McSweeney, who said he took responsibility for advising on the appointment of Mandelson to the United Kingdom's top diplomatic role in the US.
But Starmer, flanked by his cabinet, struck a defiant note, telling MPs he was staying.
"After having fought so hard for the chance to change our country, I'm not prepared to walk away from my mandate and my responsibility to my country, or to plunge us into chaos as others have done," he said.
Arguing he had changed the Crown Prosecution Service so it "better served victims of violence against women and girls" and changed the Labour Party so it could win an election.
"I have won every fight I've ever been in."
Downing Street sources characterised the prime minister as "absolutely determined" as he appeared before MPs hours after Sarwar said he should quit.
But they added he acknowledged his operation had not been "open or inclusive enough", and pledged to give more weight to the views of the Parliamentary Labour in a meeting described by MPs present as broadly positive towards Starmer.
But Sarwar, speaking in Scotland where polls suggest support for Labour has slumped since the 2024 election, said it was with a heavy heart that he had to defend Scotland and call for a change of leader in the UK capital London.
"The distraction needs to end and the leadership in Downing Street has to change," he told a press conference.
In response, a Downing Street spokesman said Starmer had "a clear five-year mandate from the British people to deliver change, and that is what he will do".
With his statement, Sarwar became the most senior Labour figure to call for Starmer's resignation, and did little to quell a febrile mood in parliament in London's Westminster.
Government borrowing costs rose, reflecting investors' concerns that a more progressive Labour leader, who was willing to borrow and spend more, could take over.
The climb in yields, along with the value of the pound against the euro, later eased after the potential successors came out in Starmer's support.
With Starmer losing his fourth director of communications, his record in government is under scrutiny, including the gaffes and policy U-turns that have tainted his almost two years in power.
"It's painful," said one Labour MP on condition of anonymity.
"It's like watching a fatal car crash in slow motion."
But Starmer received a show of support from his deputy, David Lammy, finance minister Rachel Reeves and foreign minister Yvette Cooper, among others.
Angela Rayner, his former deputy who is seen as a leading leadership candidate, offered him her "full support".
"I urge all my colleagues to come together, remember our values and put them into practice as a team. The Prime Minister has my full support in leading us to that end," she said on X.
with Reuters
