Sir Keir Starmer can prove his critics wrong and survive a leadership challenge, his friend Lord Hermer has told the BBC.
In his first interview with BBC Radio 4's Political Thinking, the attorney general said people had always underestimated the prime minister.
The pair's friendship began in 1996 when they were both barristers at Doughty Street Chambers, specialising in human rights. Sir Keir appointed Lord Hermer as the government's chief legal adviser in 2024 following his landslide victory.
But earlier this month, Labour lost control of the Welsh Senedd, as well as nearly 1,500 councillors in England, in a historically bad set of election results, leading to calls for Sir Keir to go.
Former Health Secretary Wes Streeting and a handful of junior ministers quit the government in the aftermath. Greater Manchester Mayor Andy Burnham, regarded as the frontrunner in any future leadership contest, has set his sights on returning to Westminster in the Makerfield by-election this month.
Asked whether Sir Keir believed he could survive the challenges, Lord Hermer replied: "Yes, I think he does.
"Keir has consistently been underestimated, I imagine throughout his life he has been underestimated, and he has always proven his critics wrong.
"When he was DPP [Director of Public Prosecutions], taking on some of the most dangerous organised criminal gangs, or politicians or indeed the media.
"When he came in as Labour leader, people said 'you're never gonna turn this party around so it's electable within three election cycles', but yet we had the victory in 2024, so no-one has ever got rich betting against Keir Starmer."
Burnham's first attempt to return to Parliament in February's Gorton and Denton by-election was blocked by Labour's ruling National Executive Committee. Labour lost the seat to the Green Party.
His second attempt was not blocked, and the Makerfield by-election on 18 June will put Sir Keir and Streeting in the unusual position of campaigning for Burnham to win, despite their rivalry.
Asked about Burnham's ambitions, Lord Hermer refused to engage in what he termed speculation.
"I really desperately want Andy to win and for Labour to win that election," he said.
"He is a brilliant candidate, a talented politician, the PLP [Parliamentary Labour Party] will benefit from him being in it."
The attorney general said all he knew was: "Keir is leader, prime minister, with a mandate from the general election and an absolute determination to better this country."
Lord Hermer denied Sir Keir was angry about what was happening and instead described him as the most resilient person he had ever met.
He said: "So I think, like, frankly, he'd have lots of good reason to be angry with what's going on, of course, but that's not what I pick up from him."
Lord Hermer said he disagreed with former Labour Prime Minister Sir Tony Blair's criticism of Sir Keir having a lack of vision, insisting that the prime minister had a clear vision for the country, based on equality and respect for everyone, regardless of their background.
"What that means in real terms is that everybody in life has to be given a fair crack of the whip," he said.
"It doesn't matter the wealth of your parents, it doesn't matter the circumstances in which you were born, you should be able to realise your potential in life."
A full list of the candidates for the Makerfield by-election can be found here.
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