The Conservatives have suffered significant losses at Thursday's local elections, losing almost 500 seats and control of 11 councils.
The party shed support to the Liberal Democrats in southern England, and lost key London councils to Labour.
The Tories also saw losses in Scotland, where the SNP won the most seats, and in Wales to a resurgent Labour. The PM said it had been "tough" in some areas.
Sinn Féin is on track to win the most seats in the Northern Ireland Assembly.
The party won the most first-preference votes, and has the largest number of candidates returned so far, with 18 of the 47 seats declared. More results are to follow on Saturday.
The results come after votes to elect 200 councils in England, Wales and Scotland, and the devolved Stormont assembly in Northern Ireland.
Prime Minister Boris Johnson argued the results had been mixed overall.
Labour gains were more modest outside London, with limited breakthroughs for the party in its traditional heartlands in northern England.
With nearly all results in, Labour has gained 139 seats, with leader Sir Keir Starmer saying his party was "on track" to win the next general election.
But his day was overshadowed by news that police are investigating whether he breached Covid rules at an event in Durham last year.
The BBC projects Labour would take 35% of votes and the Tories 30%, had all parts of Britain gone to the polls.
The projection gave Labour its biggest local election lead in a decade, and could lead to renewed criticism of Mr Johnson within his own party.
The findings - based on the results declared so far from areas which voted on Thursday - put the Liberal Democrats on 19% and other parties on 16%.
In the BBC's collection of key wards, the Conservative vote is down on average by seven points since 2018 in the south of England, compared to four points in both London and the Midlands and just two points in the North.
A valuable measure
The local elections don't hold up a perfect mirror to the whole country's desires, or exactly what would happen if a general election had just happened.
But they are a valuable measure of the state of the battle of the two main parties who seek to govern us all. Caveats aside, the events of the last 24 hours do matter.
Spin back a few weeks, and some Conservatives feared these elections would end in absolute calamity after months of extraordinary revelations about what had been going on in Downing Street during lockdown.
Now that nearly everything has been counted, the Tories have lost nearly 500 seats across the UK. It represents a rejection of the Conservatives in many parts of the country they see as home.
The losses are without question at the worse end of pundits' expectations.
Yet right now it doesn't seem like the kind of wipe-out that will explode Number 10. And after 12 years in government, losses like this in a mid term are not without precedent.
Ahead of the elections, the Conservatives had been braced for big losses in England, as they faced attacks over the rising cost of living and the ongoing Partygate scandal, including the prime minister's fine for breaking lockdown rules.
Speaking on a visit to Ruislip, north-west London, Mr Johnson said: "It is mid-term. It's certainly a mixed set of results.
"We had a tough night in some parts of the country, but on the other hand, in other parts of the country you are still seeing Conservatives going forward and making quite remarkable gains in places that haven't voted Conservative for a long time, if ever."
Although Conservative losses are not as bad as some in the party had warned, there was still criticism of Mr Johnson from his own side, with several Conservative council leaders blaming him after they suffered defeats locally.
Visiting Barnet in north London earlier, Sir Keir told party supporters: "This is a big turning point... From the depths of the 2019 general election, we're back on track."
"We've sent a message to the prime minister: Britain deserves better," he added.
Liberal Democrat leader Sir Ed Davey said the results would be a "turning point" for his party's fortunes.
He added that voters had "had enough" of the Tories amid the squeeze on living standards, and his party were the "real challengers" in Conservative-held areas.
Winners and losers
The first results were declared in England, where the Liberal Democrats saw almost new 200 councillors voted in, taking control of Woking, Gosport and Hull councils.
The Greens performed well too, gaining 84 seats across Britain overall, most of which were in England.
For Labour, significant results came in London, where they took the councils of Wandsworth, Westminster and Barnet, ending decades of Conservatives control in each.
In Scotland, Labour won control of West Dunbartonshire and overtook the Conservatives, but was a distant second to the SNP, which increased its councillors by 22 to 453.
SNP leader Nicola Sturgeon, the country's first minister, said the result was a "quite incredible outcome" for her party after 15 years in government.
In Wales, the Conservatives lost 86 seats and overall control of Monmouthshire, their only council there.
Labour remains the biggest party in Wales, while nationalist party Plaid Cymru gained three councils with strong results.
Welsh Labour leader Mark Drakeford said it had been a "good day for Labour" while Senedd Conservative leader Andrew RT Davies said he accepted things "might not have gone our way".
In Northern Ireland, all 90 members of the assembly are up for election.
The result could herald a historic political shift if Sinn Féin - a nationalist party - becomes the largest party for the first time in Northern Ireland's history.
The party has won the most seats so far, with 18 of the 47 seats declared.
So far 198 of 200 councils across England, Wales, and Scotland have declared. The last two are set to be confirmed on Saturday, when full results for the Northern Ireland Assembly are also due.