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Morgan McSweeney, the powerful chief of staff credited with masterminding Keir Starmer's rise to Downing Street, has resigned from his post - as crisis engulfed the prime minister's government.
The Cork-born strategist, one of Starmer's closest allies and a key architect of Labour's 2024 election landslide, stepped down today after claiming 'full responsibility' for Peter Mandelson being appointed as the UK's US ambassador.
McSweeney said the Mandelson appointment was 'wrong' and had 'damaged our party, our country and trust in politics itself,' adding that, when asked, he had advised Starmer to make the choice.
'In public life responsibility must be owned when it matters most, not just when it is most convenient,' he wrote, explaining his decision to quit,' he added in a statement.
McSweeney's departure is being interpreted in Westminster as a deliberate attempt to contain the fallout - a sacrificial move designed to protect Starmer's premiership as pressure mounts from opposition parties and his own party.
The Mandelson appointment has become a political albatross after new revelations linked the former Labour peer further to paedophile financier Jeffrey Epstein.
Although Mandelson was sacked from the ambassador role last year, subsequent disclosures about his past and ongoing investigations have intensified criticism of the decision to send him to Washington.
McSweeney's influence in Westminster was immense: he was widely seen as the 'power behind the prime minister's throne,' driving strategy throughout Labour's period in opposition and helping to shape the party's policy and messaging in government.
Prime Minister Keir Starmer is seen leaving 10 Downing Street in London with his chief of staff, Morgan McSweeney in May 2025
Peter Mandelson, then British Ambassador to the United States, and McSweeney leaving Number 10 in June 2025
But the fallout from the Mandelson affair has fractured loyalties. Senior Conservative figures and Labour critics have seized on the episode to demand greater accountability from the prime minister, with some suggesting that McSweeney's resignation - while significant - may not be enough to stabilise the government's standing.
Starmer has publicly thanked McSweeney for his service, praising his role in rebuilding the party and winning the election, but the loss of such a central figure at No 10 will intensify speculation about the prime minister's own future as the crisis deepens.
Born in 1977 in Macroom, County Cork, Ireland, McSweeney came from a relatively conventional Irish family.
His father, Tim McSweeney, was a senior partner in an accounting firm, while his mother, Carmel McSweeney, ran the local bridge club.
McSweeney moved to London as a youth, taking on a range of jobs such as working on building sites before immersing himself in British politics.
McSweeney's political journey began in earnest in the late 1990s when he joined the Labour Party in 1997, partly motivated by its support for the Good Friday Agreement.
He quickly cut his teeth in political organising: he worked in Labour's so-called 'attack and rebuttal unit' at Millbank, helped campaign in marginal seats during the 2005 general election, and played an instrumental role in winning back control of Lambeth Council for Labour from opposition parties.
British Prime Minister Keir Starmer (C) and Morgan McSweeney (L) with U.S. President Donald Trump (C-R), Vice President JD Vance (3rd-R), Secretary of State Marco Rubio (2nd-R) and Treasury Secretary Scott Bessent (R) look on at the White House on February 27, 2025
Downing Street Chief of Staff Morgan McSweeney arrives for the annual Lady Mayor's Banquet at the Guildhall in London, Britain, December 1 2025
Morgan McSweeney (L) and Prime Minister Keir Starmer arrive at the back entrance to Downing Street as he returns from Washington after meeting with American President Donald Trump
After the 2010 general election defeat, McSweeney became head of the Labour Group Office at the Local Government Association, gaining deeper experience in party operations and strategy.
In 2017, he co-founded and served as director of the influential Labour Together think tank, with a mission to reposition the party away from the hard left and build support for a more centrist, electable Labour platform.
His close association with Starmer began in earnest after the latter won the Labour leadership in April 2020.
McSweeney first became Sir Keir's chief of staff in opposition, helping to shape campaign strategy and internal party reform.
He later worked as Labour's director of campaigns, and after Labour's landslide victory in the 2024 general election, he was appointed head of political strategy at 10 Downing Street before succeeding Sue Gray as chief of staff in October 2024.
Throughout this period, political commentators described him as one of the most influential figures in British politics - even if he seldom appeared in public.
In June 2024, New Statesman ranked McSweeney as the most influential person shaping left-wing politics in the UK.
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