Sir Keir Starmer has insisted he will not quit as prime minister and "plunge the country into chaos", as Labour reels from significant election losses.
The results in England, Scotland and Wales have piled pressure on Sir Keir, with some Labour MPs calling for him to set a timetable for his departure, although his cabinet allies have backed him for now.
Labour has lost power in Wales, ending its 27-year rule, while the SNP remains the largest party in Scotland.
Reform UK has been the big winner in England, picking up more than 1,400 seats and taking control of councils in areas where Labour and the Conservatives have been historically dominant.
The BBC's projected national share (PNS), which is calculated from results in more than 1,000 wards and estimates a general election where people voted along similar lines, makes Reform the largest party, on 26% of the vote share.
The PNS puts the Greens in second on 18%, followed by Labour and the Conservatives neck-and-neck in third place on 17% - appearing to confirm the end of the traditional dominance of the big two parties in the UK.
Labour has lost more than 1,100 English council seats, including in its heartlands across northern England and the Midlands, and faces further difficult results as counting continues throughout Friday.
The results are bleak for Labour in Wales, where Plaid Cymru is now the biggest party in the devolved parliament, with Reform as the main opposition.
Meanwhile in Scotland, the SNP is the largest party in Scotland but fell short of an overall majority.
Labour, who not so long ago harboured ambitions of toppling the SNP, finished a distant second on 17 seats - tied with Reform, who made their electoral breakthrough in Scotland.
Labour's poor showing in the elections has fuelled further questions about Sir Keir's leadership which have been growing for months.
In a bid to head off pressure, Sir Keir has written a piece in Saturday's Guardian, vowing to stay on course and build unity.
"While we must respond to the message that voters have sent us, that doesn't mean tacking right or left," he said.
"It means bringing together a broad political movement, being assertive about our values, bold in our vision and addressing people's demands. Unifying rather than dividing."
Late on Friday, Health Secretary Wes Streeting said the prime minister would "have my support" as he sets out the party's response to the results in a speech expected on Monday.
Streeting said the government must "take responsibility" for Labour's heavy losses and show they can "still deliver...the change that people are crying out for", stressing the party must work "as a team".
Asked by the BBC if he was ruling out a leadership bid, Streeting said Sir Keir had "delivered a majority that people thought was absolutely impossible after the crushing defeat of 2019".
As of late Friday evening, 22 Labour MPs had publicly called for the prime minister to stand down or set a timetable for his exit and, privately, criticism of the prime minister goes far beyond the party's left wing.
Louise Haigh, the former transport secretary, said although the unpopularity of Sir Keir had come up on the doorstep, she felt now was not the time to move against him.
"We cannot descend into an irresponsible, messy, internal contest," Haigh told the BBC, although she added that, if the prime minister did not change his ways, he "cannot lead us into another election".
Haigh is an influential voice on Labour's so-called soft left, which includes supporters of Greater Manchester mayor Andy Burnham.
Burnham is seen as a potential challenger to Sir Keir, who blocked him from standing to become an MP earlier this year, so some MPs are thought to be biding their time until he can return to Westminster and launch a leadership contest.
In a speech on Friday morning, as initial results in England came in, Sir Keir acknowledged it had been a "tough" set of elections but said: "I'm not going to walk away and plunge the country into chaos."
Sir Keir is planning an attempt to reset his premiership next week, once the dust has settled on the elections.
In a series of posts on X on Friday evening, most cabinet ministers rallied around the prime minister.
Chancellor Rachel Reeves said Sir Keir "won a mandate to change our country", adding "we must get on with delivering that mandate - and show how politics can improve people's lives for the better".
Defence Secretary John Healey said he believed the prime minister could "still turn this around" while Foreign Secretary Yvette Cooper warned against a "knee-jerk reaction" to the results.
Energy Secretary Ed Miliband - another leading figure on the soft left - was among the last to post a message, saying "these are devastating results for Labour".
"As Keir has said, we must go further in delivering the mandate for change that Labour won in 2024 - and show how we will answer the call for change in our country," he added.
Some union leaders have urged Sir Keir to quit, with Unite leader Sharon Graham saying the "writing is on the wall" for the prime minister's government.
But four former Labour general secretaries have urged the party to focus on its plan to change the country rather than arguing about Sir Keir's leadership.
The unions that financially back Labour have called for an "urgent meeting" with the prime minister, suggesting the "disastrous election results" show a "stark disconnect between the Labour government and working people".
In a joint statement which includes even the usually more supportive unions, they are calling for a change of direction on "economic policy and political strategy".
In Wales, Plaid Cymru won the most seats in the Senedd but fell short of an overall majority, with Reform UK coming second.
It is the first time Labour has lost a national vote in Wales in more than a century, with the party reduced to a rump of just nine members in an expanded Senedd.
In a result that underlined the scale of Labour's decline, First Minister Eluned Morgan lost her seat and announced she would step down as leader of the Welsh party.
In Scotland, the SNP extended its 19-year stint in power.
In one of the biggest shocks of the day, SNP minister Angus Robertson lost his Edinburgh seat to the Scottish Greens, who said they were "buzzing" at the prospect of gaining seats.
Scottish Labour leader Anas Sarwar said the party "didn't win the argument" and stood by his previous call for Sir Keir to resign as prime minister.
Reform UK has made the biggest gains in England and party leader Nigel Farage said the results marked a "truly historic shift in British politics".
Speaking in Havering, one of the councils Reform UK took control of, Farage said the old era of Labour and Conservative domination had ended, vowing "the best is yet to come" for his party.
The Greens have also done well in England, winning dozens of councillors and the Hackney mayoralty in London.
Green Party leader Zack Polanski said the results showed two-party politics was dead and voters had rejected the prime minister.
The Liberal Democrats have seen mixed results, winning control of Stockport and Portsmouth, but lost control of Hull council.
Speaking in Portsmouth, Liberal Democrat leader Sir Ed Davey said Labour and the Conservatives had "let the country down" and the voters were tempted by "extreme" parties such as Reform and the Greens.
Like Labour, the Conservatives also lost hundreds of councillors, with Reform UK taking former strongholds such as Brentwood, Tamworth, North East Lincolnshire and Essex.
But the party had some success in London, regaining control of Westminster from Labour and becoming the largest party in Wandsworth.
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