Keir Starmer is fighting to reassert control over his party after accepting the resignation of his closest adviser, Morgan McSweeney, amid anger over the appointment of Peter Mandelson as US ambassador.After days of pressure over the scandal, his departing chief of staff said on Sunday he took “full responsibility” for his advice to send Mandelson to Washington despite his ongoing relationship with Jeffrey Epstein, which McSweeney conceded had undermined trust in Labour and in politics itself.Starmer’s allies hope the departure of one of the most powerful and divisive Labour advisers in recent political history will help quell mounting anger among MPs over his leadership – and keep potential challengers at bay.But the loss of the man seen as the architect of Starmer’s rise to power is a huge blow.The focus is now set to turn once again to the decision made by the prime minister to appoint Mandelson.

Senior Labour sources said McSweeney’s departure had left the prime minister dangerously exposed as he headed towards a series of policy and electoral challenges – including the Gorton and Denton byelection later this month – that could determine his political fate.A source close to Downing Street said: “Keir has just lost his firewall, on Mandelson and a whole load of other issues.Where does he think the anger gets directed next?”One MP said: “The buck ultimately stops with the PM.All this has done has bought him a little more time but he’s mortally wounded and it’s not if, it’s when he goes.”Senior government figures said the pressure would only increase on Starmer with the expected release of hundreds of thousands of documents, including private WhatsApp messages and emails between Mandelson, No 10 officials and ministers while he was in Washington.

One well-placed source said the documents would show that the Cabinet Office’s propriety and ethics team had warned of the serious reputational risk of appointing Mandelson given his publicly known links with Epstein.The loss of McSweeney is a blow to Starmer, who had come to rely on his political judgment, often over his own, despite No 10 regularly coming under fire as a result.The pair were understood to have had several conversations over the last few days about McSweeney’s future before reaching agreement on Sunday that he should go.Political opponents and senior figures inside the Labour party accused the prime minister of leaving McSweeney to carry the can over Mandelson’s appointment when ultimately it came down to Starmer’s own political judgment.Allies of McSweeney said it was wrong to suggest he had pushed for the appointment, with one saying: “He gave his advice, but that’s all it was.

Keir made the decision.”In a statement on Sunday, McSweeney said: “After careful reflection, I have decided to resign from the government.The decision to appoint Peter Mandelson was wrong.He has damaged our party, our country and trust in politics itself.When asked, I advised the prime minister to make that appointment and I take full responsibility for that advice.

In public life responsibility must be owned when it matters most, not just when it is most convenient.In the circumstances, the only honourable course is to step aside.”He said the vetting process for appointments needed “fundamental overhaul” after the Mandelson affair.“This cannot simply be a gesture but a safeguard for the future,” he said.Downing Street staff received an email on Sunday evening confirming Starmer had asked Jill Cuthbertson and Vidhya Alakeson, McSweeney’s deputies, to take over as acting chiefs of staff, with immediate effect.

Since Labour entered power, McSweeney has been a lightning rod for discontent among some backbenchers and grassroots members who were uncomfortable with his style and the direction the party was taking on issues including immigration and welfare cuts,A senior MP said: “It’s about time,McSweeney was behind a string of terrible decisions because he cannot behave anything other than factionally,The PM needs to grip this and bring in a much wider range of voices than those he’s listened to up to now,”A series of senior Labour politicians had lined up to criticise the No 10 operation over the weekend.

The former prime minister Gordon Brown said Starmer was a man of integrity but had been “slow to do the right things”.David Blunkett said the situation was “dire” for Starmer and that McSweeney needed to go.But the former home secretary also warned allies of Angela Rayner and Wes Streeting to stop the briefing wars.“When people see a party acting like ferrets in a sack they draw their conclusions,” he said.Others shared concerns that Starmer was not up to the task.

One former New Labour cabinet minister said: “Although there may be some satisfaction at Morgan’s resignation, my concern is the problem is Keir, not his advisers.”Friends of McSweeney told the Guardian he planned to step away from politics for a period but that he was unlikely to stay on the sidelines for good.“Morgan has been doing this for 25 years, I’m sure he’ll get more involved again in future, but not right now,” one said.McSweeney said in his statement: “This has not been an easy decision.Much has been written and said about me over the years but my motivations have always been simple: I have worked every day to elect and support a government that puts the lives of ordinary people first and leads us to a better future for our great country.

Only a Labour government will do that.“I leave with pride in all we have achieved mixed with regret at the circumstances of my departure.But I have always believed there are moments when you must accept your responsibility and step aside for the bigger cause.”Starmer said on Sunday he owed McSweeney a “debt of gratitude”.He said in a statement: “He turned our party around after one of its worst ever defeats and played a central role running our election campaign.

It is largely thanks to his dedication, loyalty and leadership that we won a landslide majority and have the chance to change the country.”The prime minister had expressed confidence in his former right-hand man as recently as Thursday.McSweeney is the second Downing Street chief of staff to stand down, after Sue Gray quit in 2024, and it is the latest in a slew of departures from No 10 in recent months.Kemi Badenoch, the Conservative leader, said the prime minister should “take responsibility” for his actions.“It’s about time.

But once again with this PM it’s somebody else’s fault: ‘Mandelson lied to me’ or ‘Morgan advised me’,Keir Starmer has to take responsibility for his own terrible decisions,But he never does,”