Nigel Farage said he would "change the shape of British politics" after his Brexit Party stormed to victory in the European elections.
Conservatives were the main casualties of the Brexit Party's advance, securing just 9 per cent of the vote in England and Wales and set 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 Brexit alternatives offered by Mr Farage and the pro-EU Liberal Democrats.
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The scale of the Tory disaster was underlined by its single-digit vote share - in fifth place behind the Brexit Party on 33 per cent, Lib Dems on 21 per cent, Labour on 15 per cent and the Greens on 12 per cent 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, overtaking the 24 MEPs that Mr Farage's former party Ukip sent to the European Parliament in 2014.
The Lib Dems, reduced to just a single MEP in 2014, were on 15 after their best ever European results.
Labour had ten, halved from 20. The Greens, which 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 today, but First Minister Nicola Sturgeon said her SNP had won "emphatically" and would take three of the six available seats.
The Brexit Party, which came second across Scotland, appear on course to secure one more MEP, as will both the Conservatives and the Liberal Democrats.
Mr Farage said he was getting ready to fight a general election, warning his Brexit Party could "stun everybody" if Britain had not left the EU by the next national contest.
"We're not just here to leave the European Union, but to try and fundamentally change the shape of British politics, bring it into the 21st century and get a Parliament that better reflects the country," he said.
READ MORE: EU Election results: Find out who won all of the seats for Scotland
Mr Farage added: "I'm off to the office now, we've got 650 candidates to interview, vet."
Asked if he could replicate the strong result in a general election, he said: "Let's put it like this: 29 March was a big, significant day, everyone knew that was the day we were supposed to leave and we didn't. 31 October will become that next date embedded in people's memory.
"If we don't leave on 31 October then the Brexit Party could stun everybody in a general election, too."
Mr Farage reacted angrily to suggestions of working with the Conservatives, dismissing leadership bid favourites for backing the Prime Minister's deal.
"Why would I work with a party I don't trust? Why would I work with a party who have openly, wilfully lied to the British people?" he said.
"If a new Tory leader says 'we are leaving on 31 October,' well I'll be thrilled to hear it, but do you know something? I'll believe it when I see it.
"All I can see at the moment is a series of front-runners, all of whom backed Mrs May's new European treaty. That's not good as far as I'm concerned.
"Mr Johnson, along with Mr Gove and the others voted for the new European treaty that would've left us with Brexit in name only."
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.
Home Secretary Sajid Javid, another potential leadership contender, said the "hugely disappointing" results were a "clear lesson" the public wants the Government to get on with delivering Brexit.
Prominent Brexiteer and MEP Daniel Hannan, who managed to cling on to his seat in the South East, said 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."
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."
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."
Votes from Northern Ireland were being counted today.
The results across Europe suggest a decline in the main centre-right and centre-left groupings in the European Parliament, with a boost for the Liberals, Greens and nationalists.