Rishi Sunak has said he accepts responsibility for the Conservative Party's catastrophic general election defeat.
Sir Keir Starmer has led the Labour Party to a landslide victory and will take over from Mr Sunak as the UK's prime minister.
Mr Sunak told supporters: "The British people have delivered a sobering verdict tonight, there is much to learn... and I take responsibility for the loss."
Speaking in central London, Sir Keir said "change begins now", adding "it feels good, I have to be honest".
With nearly all results declared, Labour is projected to form the next government, with a majority of 170.
The Tories are set for the worst result in their history. They have lost more than 248 seats and are currently on 119 seats. Former
Prime Minister Liz Truss - whose brief, disastrous time in office led to a slump in Tory support from which it never recovered - lost her South West Norfolk seat to Labour by 630 votes.
Ms Truss saw her huge 32,988 majority overturned, with the Reform candidate coming third with 9,958 votes.
She is among dozens of senior Tories who have lost their seats, including Defence Secretary Grant Shapps, Commons leader Penny Mordaunt, Justice Secretary Alex Chalk and former minister Sir Jacob-Rees Mogg.
Foreign Secretary James Cleverly told the BBC a "large number of people who had previously voted Conservative have voted Reform" and the Conservatives now had to "think hard" about how to win back their support.
Reform UK leader Nigel Farage won a seat in Parliament at his eighth attempt, in Clacton, promising "this is just the first step of something that is going to stun all of you".
Reform has four MPs so far - including chairman Richard Tice and former Tory Lee Anderson - and has finished second in many parts of the country, taking large amounts of votes from the Conservatives.
In a victory speech in London, Sir Keir told cheering Labour supporters the country was waking up to "the sunlight of hope" which was "shining once again on a country with the opportunity after 14 years to get its future back".
He added: “Now we can look forward – walk into the morning.”
The Liberal Democrats have slightly fewer votes than Reform but have benefitted most from the Tory collapse, surging to a record 70 MPs, including the constituencies of three former Tory PMs - Boris Johnson, David Cameron and Theresa May.
Lib Dem leader Sir Ed Davey said: "This is a record-breaking night for the Liberal Democrats."
He added: "We will now work hard to keep that trust with a focus on the issues that matter most to them, most of all the NHS and care."
The Green Party of England and Wales now has four MPs, with co-leaders Carla Denyer and Adrian Ramsay among the winners.
But it has been a terrible night for the SNP, which has been reduced to just eight MPs so far.
Former Labour leader Jeremy Corbyn has defeated his old party to retain his Islington North seat as an independent.
But another high profile former Labour MP, George Galloway, failed to retain the Rochdale seat he won at a by-election in February, losing to Labour's Paul Waugh.
Sir Keir Starmer's landslide is short of the 179 majority won by Tony Blair in 1997, with its vote share across the country up by just 2%, largely thanks to big gains in Scotland, according to polling expert Sir John Curtice.
But it will mean a Labour prime minister in Downing Street for the first time since 2010 and a battle for the future direction of the Conservatives if, as seems likely, Rishi Sunak stands down as leader.
