Starmer is on the back foot after his Labour Party recorded the worst losses of a governing party in municipal polls since 1995, prompting a growing number of his own MPs to call on him to quit.
Aiming to reset his leadership and win back party support, Starmer's office announced the appointment of two Labour grandees to his team as advisers.
Brown, 75, will join as an adviser on global finance and co-operation, while former Labour deputy leader Harriet Harman, 75, was appointed to the role of the prime minister's adviser on women and girls.
As Tony Blair's finance minister, Brown was a key architect of the New Labour project that won the party three consecutive general elections from 1997.
Serving as prime minister himself from 2007 to 2010, Brown was instrumental in nationalising major banks and stabilising the financial system during the global financial crisis.
Starmer vowed to stay on as leader on Friday as the scale of his party's defeat started to emerge.
Labour losses stood at 1417 seats as the final votes were counted on Saturday, a bigger defeat than the 1330 seats lost by former prime minister Theresa May's Conservative Party in 2019.
May quit three weeks after that result.
While an immediate challenge to Starmer's leadership does not look likely and ministerial allies have signalled their support for him, there are growing calls for him to resign.
More than 20 MPs publicly and privately have already called on him to consider his position and set out a timetable for his departure, with former minister Catherine West joining the fray on Saturday.
"His approach is not cutting through, and the results over the past 48 hours are nothing short of disastrous," West said of Starmer on X.
"I know I speak for more Labour people than just myself in wanting him to step aside as our Leader."
Just under two years after leading Labour to a landslide national election victory, voters have quickly turned against Starmer.
Against the backdrop of a cost-of-living crisis compounded by conflicts in Ukraine and Iran, his government has been beset by policy U-turns, a rotating cast of advisers and scandal over the appointment of another Blair-era veteran, Peter Mandelson, as Britain's ambassador to the United States.
Starmer, writing in the Guardian newspaper on Saturday, said that while his government had made "unnecessary mistakes", he was focused on "building a stronger and fairer country" as he promised to respond to the message from voters.
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