As voters in Scotland and Wales elect their national parliaments and 5,000 seats are contested in local elections in parts of England, the pressure is on beleaguered Labour Prime Minister Keir Starmer, with right-wing populists Reform and the Greens predicted to make gains.
Polls opened at 7am across Scotland, England and Wales in ballots that will be Starmer's biggest electoral test since his July 2024 general election landslide victory ended 14 years of Conservative rule.
Opinion polls predict grim results for Labour, which could amplify calls for Starmer, 63, to resign or face a long-rumoured party leadership challenge.
Nigel Farage's anti-immigration Reform UK and the left-wing Greens, led by self-described "eco-populist" Zack Polanski, are expected to be the main beneficiaries of widespread disillusionment.
"It's a huge barometer for how the country is feeling about this political establishment," said Melanie Garson, associate professor of politics at University College London.
"We've got, for the first time, significant pressure on the main political parties across every single council."
Polls will close at 10pm. Some results are expected overnight, but most will not come until later on Friday.
Around 5,000 local council seats, out of 16,000, are up for grabs across England, while in Wales and Scotland voters will elect new parliaments.
Starmer swept to power two years ago following 14 years of largely chaotic Conservative rule defined by austerity, Brexit and the tanking of the economy under former prime minister Liz Truss.
But critics say he has swerved from one policy misstep to another, and he has been embroiled in a scandal over Peter Mandelson, who was sacked from his role as UK ambassador to the United States over his ties to convicted sex offender Jeffrey Epstein.
Starmer has also failed to fulfil his main promise of spurring economic growth, with impatient Britons still suffering a cost of living crisis, including from high energy prices.
"The change hasn't been delivered, or change that has been delivered has been negative," Garson told AFP.
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Leadership challenge
Starmer said on Wednesday there was a "clear choice" at the ballot box: "Unity or division. Progress versus the politics of anger."
Labour has also fought back, unearthing racist remarks by some Reform candidates and anti-Semitic comments by certain Green hopefuls.
But Starmer is now one of the most unpopular prime ministers in UK history, and surveys suggest Labour will lose control of the devolved Welsh government in Cardiff for the first time since Wales got its own parliament 27 years ago.
A More in Common poll published on Tuesday projected Reform as neck and neck with the pro-independence Plaid Cymru in Labour's former heartland.
Labour is also fearful of a drubbing in Scotland, where the Scottish National Party is expected to extend its 19-year control of the devolved parliament in Edinburgh. YouGov has predicted Reform could even force Labour into third place there.
"The message is clear: if you want real change, you'd better vote for it, and we go into tomorrow feeling pretty optimistic about our prospects," Farage said.
Labour also looks set for big losses in London, as the Greens pick up disaffected left-wingers in urban areas with a pro-Gaza message.
Pollster Robert Hayward has predicted the UK's ruling party could lose around 1,850 of the roughly 2,550 English local authority seats it is defending.
Hayward has tipped Reform to take 1,550 seats from Labour and Kemi Badenoch's right-wing Conservatives – mostly in white, working-class areas. The Conservatives are also bracing for the loss of traditional strongholds.
"The two-party era has moved into a multi-party era," Badenoch told British news agency PA. "But the fact is none of these new parties or Labour have a plan for the country."
UK media is full of rumours that former deputy prime minister Angela Rayner or Health Secretary Wes Streeting could try to oust Starmer after the results.
Neither is universally popular within Labour, however, and would need the backing of 20 percent of the party's MPs to launch a contest.
Some Labour MPs are also reportedly planning to demand that Starmer set a date for his departure.
He has insisted he will lead the party into the next general election, likely to be held in 2029.
