The former deputy Labour leader said the prime minister "seems to have drifted so far away from [his] values in appointing Peter Mandelson" to be US ambassador, and that he must return to them to survive.
It follows growing anger within the Labour Party at the PM's initial decision to oppose releasing documents about the vetting and appointment of Lord Mandelson, which led to a public climbdown last night in the Commons.
Speaking to the Electoral Dysfunction podcast, Harriet Harman also warned the increasing angst around the PM's handling of Lord Mandelson is "so serious" and has called for Sir Keir's chief-of-staff, Morgan McSweeney, to go if he was behind the appointment.
Baroness Harman told Sky News' political editor Beth Rigby: "I think it is so serious for Keir Starmer. I don't think it's inevitable that it will bring him down.
"But it will bring him down - unless he takes the action, which is really necessary for him to take, and that's this: firstly, he's got to stop blaming Mandelson and saying, 'he lied to me'. Because actually he should never have been considering him in the first place.
"To say 'he lied to me' makes it look weak and naive and gullible. So it's just completely the wrong thing.
"He should be reflecting on why he made that appointment, not angry at the evilness of Peter Mandelson.
"He should also be thinking about a real reset in Number 10, because what you need from your team in Number 10 is people who share your values and your principles and who will help you be the best prime minister you can be, according to your true self.
"Clearly, that is not what happened because the Keir Starmer, who was DPP [director of public prosecutions], would never have appointed somebody like Peter Mandelson to represent the country."
The Labour peer also insisted that Sir Keir must "go back to the manifesto and do all the things that were promised in cleaning up politics and... bring them forward".
This includes bringing forward the government's action on tackling violence against women and girls, Baroness Harman said.
"If he does that, he can win the confidence of Labour backbenchers, the confidence in parliament and the confidence for the country," she said. "But I think that if he doesn't do that, there's no way this is going to be glossed over."
She added that she wants both the government and Sir Keir to succeed, and pointed out how she knew him well when he was the DPP.
Baroness Harman said: "If you'd asked me what he stood for, I would have said standing up for human rights, standing up to, for victims, standing up for women and girls. Standing up for decency and honesty.
"And yet he seems to have drifted so far away from those values in appointing Peter Mandelson to be the UK's representative in the US.
"Peter Mandelson was called the Prince of Darkness. It's not a secret that he was a bad person. I think that it is very, very serious for Keir Starmer because it goes to the values of the government."
You can listen to the full episode of Electoral Dysfunction here on Friday morning.
A man heckling about Andrew Mountbatten-Windsor in an apparent reference to the Jeffrey Epstein scandal, and a reporter asking a question, neither getting any kind of response from the King.
The monarch was dodging the questions and the wet weather in Dedham.
He's been in this position many times over the years, scandals threatening to overshadow his engagements.
His approach - and I've seen it on many occasions - is don't answer and don't give the story further oxygen.
A man in the crowd, who was wearing a grey hat and holding a blue umbrella, shouted as the King was near him: "Charles, Charles, have you pressurised the police to start investigating Andrew?"
Shortly afterwards a news reporter in the crowd - who was beside a cameraman - apparently tried to ask Charles a question about his brother.
The shouts are a story but his response, should he give one, would be much bigger.
That will frustrate those who believe the monarch should say more, that the institution has questions to answer about Andrew and the Epstein files.
From speaking to palace sources, they're clearly keen to make sure the public don't think they're burying their heads in the sand on this.
Like all of us they can't help but watch developments closely, especially when they involve one of their own.
In some ways, the fact Prince Edward was bounced into answering a question about it, while on an official trip in Dubai, did them a favour.
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He could again publicly reiterate the new palace line that the focus should be on remembering the victims, repeating an important part of the palace statement released back in October when the King stripped Andrew of his home and titles.
On Thursday, his priority would have been the hundreds of people who were braving the cold and the rain to see him and the Queen, and celebrating the community of Dedham.
Largely a crowd who believe that he's done the right things so far and that his brother has left him in a really tricky position, a crowd who also didn't appreciate the King being shouted at.
Like the monarch they didn't want Andrew to distract from their big day, but the enormity of the Epstein scandal always meant that was in many ways unpreventable.
The assault happened at a school in Milford Haven, Pembrokeshire, Dyfed-Powys Police said.
Officers said they responded to a report on Thursday afternoon that a teacher had been assaulted by a pupil.
They said the teacher was injured, but it was not a stab injury.
Superintendent Chris Neve said: "We can confirm police are at Milford Haven Comprehensive School following a report of the assault of a teacher by a pupil brandishing a weapon at the school at approx 3.20pm.
"The teacher's injury is not a stab injury. A lockdown was implemented but has now been lifted.
"The teacher is receiving medical treatment for their injuries."
Supt Neve added that "all pupils at the location are safe and most have gone home".
Officers remained at the school.
Supt Neve also said: "A 15-year-old boy has been arrested on suspicion of attempted murder and is in police custody."
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In a statement, a spokesperson for Pembrokeshire County Council confirmed police attended the school following "a report of the assault of a teacher by a pupil at the school".
"All pupils at the location are safe, and most have left the school. Police officers remain at the site. Any planned after school events have been cancelled this evening," they said.
A Welsh Ambulance Service spokesperson said: "We were called today (Thursday, 5 February) shortly after 3.20pm to a medical emergency at Milford Haven Comprehensive School.
"We sent one Advanced Paramedic Practitioner to the scene, and one patient was transported to hospital."
Deividas Skebas, 26, knifed Lilia Valutyte in the heart as she played with a hula hoop in Boston, Lincolnshire, on 28 July 2022.
Skebas, a Lithuanian who has schizophrenia and moved back to the UK just weeks before the attack, had denied murder but admitted manslaughter by diminished responsibility.
Lilia's mother, Lina Savickiene, said she had found her daughter "covered in blood and with the hoop around her".
She initially thought "something might have happened" with the hula hoop and described shouting for help while trying to cover her daughter's wounds as she became pale.
An off-duty police officer tried to help save Lilia but their efforts were unsuccessful.
Skebas was judged mentally unfit to stand trial in 2023, but that assessment changed in spring 2025 and a criminal trial began at Lincoln Crown Court last week.
Jurors were told there was no dispute he had killed Lilia but they had to decide what his state of mind was.
Prosecutors said he had known what he was doing in stabbing Lilia - who would have been 13 this week - and tried to avoid being caught.
Opening the case, Christopher Donnellan KC told jurors: "This deliberate murder was clearly a wicked act. He knew his conduct was wrong. He knew he was killing a child."
CCTV showed Skebas buying a knife two days before, as well as circling the area and launching the fatal attack before fleeing.
The Crown Prosecution Service said the video evidence was crucial in showing his judgement was not impaired.
The killer also shaved off his beard after the murder, and phone evidence showed him making arrangements to leave the UK, including asking family to lend him money to travel.
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Defence barrister Andrew Campbell-Tiech KC told the court that Skebas, who appeared via videolink from the high security Rampton Hospital, was "quite obviously deluded".
The court heard he had told police he had "the power to resurrect" Lilia if they contacted "his controller in Nasa".
Skebas was found guilty by 11-1, after over seven hours of jury deliberations. He will be sentenced on 25 February.
"During these nine years we lived life fully, we visited many places, we didn't stand still," said Lilia's mother in a statement.
"This is not something you recover from," she added.
"Sometimes terrifying thoughts overwhelm the mind and during this trial there have been many, many more."
"Why her? Why us? The questions remain unanswered," she said.
Lilia's stepfather, Aurelijus Savickas, described Lilia as a "beautiful soul" with a "strong character", adding: "Lilia, you will always live in our hearts, you are forever loved, forever missed."
Leicester, who play in the Championship, were docked the points after being found to be in breach of the English Football League's (EFL) financial rules, the Premier League said in a statement on Thursday.
The club overspent "by £20.8m over the three-year assessment period (2022-2024)," investigators said.
The commission also found the club breached Premier League rules in failing to provide their annual accounts to the league when asked to do so.
The punishment, recommended by an independent Disciplinary Commission and imposed by the EFL, relates to Leicester City's promotion to the Premier League in 2024.
The deduction puts the Foxes, who were 17th with 38 points before the punishment, just above the relegation zone in 20th place on 32 points.
The commission was appointed under Premier League rules last May, after an arbitration panel ruled the investigation could be transferred from the EFL to the Premier League following the Foxes' promotion to English football's top division in 2024.
The Premier League initially recommended a 12-point deduction.
The club avoided a points deduction in relation to alleged breaches of the top flight's profitability and sustainability rules (PSR) for the 2022-23 season after an appeal by Leicester was upheld in September 2024.
Leicester said in a statement on its website that it acknowledged the commission's decision "with disappointment" and is considering "next steps".
It said the punishment was "disproportionate", while accepting that the commission's findings "significantly reduced the unprecedented scale of the sanction originally sought by the Premier League".
The points deduction "does not adequately reflect the mitigating factors presented, the importance of which cannot be overstated, given the potential impact on our sporting ambitions this season".
In a statement, the EFL noted the commission "has recommended a deduction of six points," it said, adding that the EFL Board, which had met to consider the decision, had "determined that the sanction should be applied to the Championship table with immediate effect".
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Explaining its decision, the commission said that the club, which won the Premier League in 2016, raised a range of legal challenges over whether the rules should apply and the commission's ability to impose a sanction on the club, "all of which were rejected".
Among the objections raised during a week-long hearing in November was the claim that "the relevant rules were in breach of competition law," the commission said in its statement.
It said it accepted parts of the club's case, relating to the length of the assessment period and "that a particular accounting policy relating to player costs should apply to reduce the club's losses in its 2022 annual accounts".
"Having done so, it determined that the club had breached the relevant PSR threshold by £20.8m over the three-year assessment period (2022-2024)."
The club's claim that it had "demonstrated exceptional cooperation throughout the proceedings" was dismissed, but its improving financial position over the relevant assessment period was accepted in mitigation.