Morgan McSweeney, the political strategist who engineered Keir Starmer's rise to power and served as the prime minister's most trusted adviser inside Downing Street, resigned on Sunday. He said he took "full responsibility" for advising Starmer to appoint Peter Mandelson as Britain's ambassador to the United States in late 2024 — a decision now at the center of the gravest crisis of Starmer's 18 months in office.

"The decision to appoint Peter Mandelson was wrong," McSweeney said in a statement. "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."

He added: "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."

McSweeney's deputies, Jill Cuthbertson and Vidhya Alakeson, will immediately step up as acting chiefs of staff. Starmer emailed No. 10 colleagues with the news Sunday evening, crediting McSweeney as a central figure in Labour's landslide election victory but making no direct mention of the Mandelson scandal.

As late as Sunday morning, cabinet minister Pat McFadden had publicly argued that McSweeney should not have to leave over the appointment. That the government reversed course within hours spoke to the scale of pressure bearing down on Starmer from within his own party.

What Mandelson did — and what Starmer knew

The crisis traces back to December 2024, when Starmer appointed Mandelson, 72, a former cabinet minister and Labour elder statesman, to Britain's most important diplomatic post. The appointment was already controversial given Mandelson's known association with Jeffrey Epstein, the convicted sex offender who died in a New York jail in 2019.

Starmer has acknowledged that the vetting process at the time revealed Mandelson's friendship with Epstein had continued after Epstein's 2008 conviction for soliciting prostitution from a minor. But the prime minister maintained that "none of us knew the depth of the darkness" of that relationship.

Mandelson was fired from the ambassadorship in September 2025 after further details of his ties to Epstein became public. The situation escalated sharply in recent weeks when the U.S. Department of Justice published additional emails showing the extent of the relationship. Those documents suggested Mandelson had forwarded sensitive internal government financial discussions to Epstein while serving as Britain's business secretary during the 2008 financial crisis. Records of payments totaling $75,000 in 2003 and 2004 from Epstein to accounts linked to Mandelson or his husband, Reinaldo Avila da Silva, also surfaced.

Metropolitan Police officers searched Mandelson's London home and a second property linked to him on Friday. Police said the investigation, which centers on potential misconduct in public office, is complex and will require "a significant amount of further evidence gathering and analysis." Mandelson has not been arrested or charged, and he is not accused of any sexual offenses. He resigned his Labour Party membership and his seat in the House of Lords following the latest disclosures.

Starmer apologized this week for "having believed Mandelson's lies" and promised to release government documents related to the appointment, which he said would show that Mandelson had misled officials during the vetting process. The government has not yet published those files.

The man behind the curtain

McSweeney, 48, was not a household name. He never spoke publicly, avoided cameras and photographers, and operated almost entirely behind the scenes. But within Labour and Westminster, he was understood to be the most consequential figure in the party's transformation over the past decade.

An Irishman who cut his teeth in local government and at Labour headquarters during the 2001 election, McSweeney had spent the late Corbyn years running the Labour Together think tank, polling party members, and identifying Starmer — then the shadow Brexit secretary — as the candidate most likely to wrest control of the party back from its left flank. In what some insiders have described as a meticulously executed internal campaign, McSweeney helped persuade pro-Corbyn members that Starmer was ideologically sympathetic to their cause. Once Starmer won the leadership, the party purged many Corbynites, including former leader Jeremy Corbyn himself, and pivoted toward the center for the general election.

McSweeney became chief of staff in October 2024 after the resignation of Sue Gray, who left amid a row over pay and donations. He was widely credited as the architect of Labour's commanding parliamentary majority. He was also blamed by critics for turbulence and policy reversals once in government, and accused by some MPs of presiding over what they described as a "boys' club" atmosphere inside No. 10.

Allies of McSweeney pointed out after his departure that he was not the only adviser who recommended Mandelson for the ambassadorship, and that others who gave the same counsel remain in their posts. "He has taken full responsibility for the advice but he was not in charge of the vetting and he was not the only adviser," one McSweeney loyalist said. "Morgan was duped like many of us."

Others were less sympathetic, noting that Mandelson's post-conviction contact with Epstein was hardly hidden information. That the vetting process failed to flag it adequately — or that the flag was overridden — remains an open question the government has not fully answered.

The pressure on Starmer

McSweeney's resignation does not end the crisis. If anything, it intensifies the central question now hanging over British politics: if the man who gave the advice should resign, why not the man who made the decision?

Kemi Badenoch, leader of the opposition Conservative Party, said Sunday that McSweeney's departure was overdue and that "Keir Starmer has to take responsibility for his own terrible decisions." Nigel Farage, leader of the populist Reform UK party — which currently leads in national polling — said he believed Starmer's time would "soon be up."

The most dangerous pressure, though, is coming from inside Labour itself. Some MPs are openly questioning Starmer's judgment and his future. One Labour lawmaker, speaking anonymously, said McSweeney's resignation was "too late" and that while it "buys the PM time," it is "still the end of days." Baroness Harman, typically a government loyalist, warned on the Electoral Dysfunction podcast that Starmer could not simply blame Mandelson for misleading him. She argued he needed to clear out those who advised the appointment, deliver a genuine program to clean up politics, and bring forward action on violence against women and girls.

Former Prime Minister Gordon Brown, while backing Starmer as a "man of integrity," said the situation was "serious" and suggested the Labour leader had been "too slow to do the right things."

The 37 Scottish Labour MPs are drawing particular attention. Many privately believe Starmer is a drag on their chances ahead of Scottish Parliament elections in May, and No. 10 is planning a charm offensive starting with a reception at No. 11 on Monday and a strategy session at No. 10 on Tuesday. Starmer is also expected to address the parliamentary Labour Party on Monday night and outline steps to deliver on his manifesto pledge to "clean up politics."

What Starmer lost

Whether voters notice the resignation of a political operative most of them have never heard of is debatable. One Labour MP told reporters that talk of Mandelson "very rarely came up on the doors" during canvassing over the weekend.

But what Starmer lost Sunday goes beyond a scalp offered to quiet restless backbenchers. McSweeney was his best campaign strategist, his most trusted political adviser, and the person who had shaped his political identity from the beginning. Unlike most relationships between a prime minister and a chief of staff, it was arguably McSweeney who chose Starmer — not the other way around.

Now the prime minister faces an unresolved police investigation into his former ambassador, a party demanding a reset far larger than one personnel change, an opposition smelling blood, and a public that polls show has already lost confidence in his leadership. He will have to navigate all of it without the man who, for the better part of a decade, told him how.