The Conservatives were routed by Nigel Farage's Brexit Party in a dramatic set of European election results.
The Tories dropped to just 9% of the vote in England and Wales and appear on course for their worst ever national election share once the full picture from Scotland becomes clear.
Labour also suffered a terrible night as voters split between the clear alternatives offered by Mr Farage and the pro-EU Liberal Democrats.
The scale of the Tory disaster was underlined by its single-digit vote share – in fifth place behind the Brexit Party on 33%, Lib Dems on 21%, Labour on 15% and the Greens on 12% after all results were in from England and Wales.
Just three Conservatives were elected in England and Wales, while the Brexit Party had 28 seats, overhauling the 24 MEPs that Mr Farage's former party Ukip sent to the European Parliament in 2014.
The Lib Dems, who were reduced to just a single MEP in 2014, were on 15 after their best ever European results.
Labour had 10, halved from 20, the Greens – who also enjoyed a boost from pro-EU voters – were on seven, up from three in 2014.
The result in Scotland will not be formally declared until later on Monday but First Minister Nicola Sturgeon said her SNP had won "emphatically" and would take three of the six available seats.
The Brexit Party, who came second across Scotland, appear on course to secure one more MEP, as will both the Conservatives and the Liberal Democrats.
Mr Farage, who was elected for his new party in the South East, said: "History has been made. This is just the beginning."
In a warning to the Westminster parties, he said: "If we don't leave on October 31 then the scores you have seen for the Brexit Party today will be repeated in a general election, and we are getting ready for it."
Mr Farage repeated his demand for the Brexit Party to be involved in the negotiations ahead of the Halloween deadline.
Foreign Secretary and Tory leadership hopeful Jeremy Hunt said the dire results for the Conservatives meant the party faced an "existential risk" unless it delivered Brexit.
Prominent Brexiteer and MEP Daniel Hannan, who managed to cling on to his seat in the South East, told the Press Association it was "without question our worst result as a party ever".
But Labour also threatened to tear itself apart with a bitter row at the top of the party about the strategy adopted by Jeremy Corbyn.
The Labour leader hinted he might be prepared to change course after coming under pressure to fully support a second referendum.
Two of Labour's most senior figures – Emily Thornberry and Tom Watson – tore into the party's campaign, claiming it had lacked a clear message as it sought to appeal to both Leave and Remain voters.
Mr Corbyn said: "With the Conservatives disintegrating and unable to govern, and parliament deadlocked, this issue will have to go back to the people, whether through a general election or a public vote."
In a sign that he could consider a shift in position, he added: "Over the coming days we will have conversations across our party and movement, and reflect on these results on both sides of the Brexit divide."
In a sign of how embarrassing the results were for Labour, the Lib Dems topped the poll in Islington, in north London – where both Mr Corbyn and Ms Thornberry are MPs.
Shadow foreign secretary Ms Thornberry told the BBC: "We should have said, quite simply, that any deal that comes out of this government should be put to a confirmatory referendum and that remain should be on the ballot paper and that Labour would campaign to remain."
Instead, the party got a "kicking" because "we went into an election where the most important issue was 'what was our view on leaving the European Union' and we were not clear about it".
Deputy Labour leader Mr Watson said: "Following the disastrous EU election results, Labour urgently needs to re-think its Brexit position and realign with members and voters."
The Liberal Democrats attracted Remain-supporting voters from across the political spectrum, including former Labour spin doctor Alastair Campbell and former Tory deputy prime minister Lord Heseltine.
Lib Dem leader Sir Vince Cable said: "Our clear, honest, unambiguous message has won us our best ever European election result, and pushed Corbyn's Labour into third place.
"We have shown ourselves to be the strongest Remain force in British politics."
He said the results were a message for Labour to "get off the fence" over Brexit.
The Green Party finished above the Conservatives for the first time in a national election.
Co-leader Sian Berry said: "There is clear evidence from this of strong support for the UK remaining in the European Union, but also for tackling the causes of Brexit – the massive damage done to so many communities by austerity, tax-dodging and diminution of workers' rights."
The result from Northern Ireland is expected to be declared later on Monday.
— European Parliament (@Europarl_EN) May 26, 2019
The results across Europe suggest a decline in the main centre-right and centre-left grouping in the European Parliament, with a boost for the Liberals, Greens and nationalists.