Nigel Farage claimed victory tonight as Ukip appeared on track to top the European Parliament polls in England, declaring the result was an "earthquake" in British politics.
He said: "Ukip is going to win this election and yes that will be an earthquake because never before in the history of British politics has a party that will be seen to be an insurgent party ever topped the polls in a national election."
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The other three main parties have conceded Ukip will win the election, after the first results were announced.
In the eastern region Ukip was up two to three MEPs, with the Tories and Lib Dems each losing one. The region returned three Tory MEPs and one from Labour.
In the North East Labour returned two MEPs, up one, with Ukip returning a single MEP, also up one. Again the Conservatives and Lib Dems lost a seat each.
David Cameron, Ed Miliband and Nick Clegg have all faced criticism over the way they have responded to Ukip's rise and strategists will study the results of the final national ballot ahead of next year's general election as they plan how to tackle a new era of four-party politics.
As polls closed across Europe, Conservative Party chairman Grant Shapps acknowledged his party would lose seats as results emerged.
He said: "I think it is going to be an interesting night to watch... we are seeing results coming in across Europe with a very clear message to the elite in Europe that people want to see change.
"The Conservative Party has long since got that and that is why we want a referendum following renegotiation."
Tory Europe Minister David Lidington accepted that "Ukip will probably come top of the poll".
He paid tribute to ousted Tory MEP Martin Callanan, who was leader of the European Conservatives and Reformists grouping in the European Parliament who represented the North East.
"You have a guy there who has fought really hard for British interests and I think he will be a loss to the North East and to the country," he said.
Chief Secretary to the Treasury Danny Alexander did not rule out the Liberal Democrats finishing fifth behind the Green Party - but said he expected to retain some MEPs.
"I do not know what is going to happen. We will have to see. There are regions where we will do better than in the North East," he said.
"Some people have been predicting that we will end up with no MEPs at all. I suspect we will end up in the very low single figures."
Ukip's chief spin doctor Patrick O'Flynn, elected as an MEP in the Eastern region, told LBC: "I think we have got a very high calibre of candidates.
"Everyone who has got elected in the UK as an MEP must think profoundly about why they have been elected and why they have been sent there."
Mr O'Flynn said there had been "brickbats" thrown at Ukip during the campaign by some newspapers, but added: "It does not seem to have harmed us.
"We have a right to claim we are somewhat closer to their readers than some of those newspapers."
Ukip came top in Eastern region with 542,812 votes to the Conservatives' 446,569.
Mr O'Flynn, the party's communications director, said Ukip must now build for the future ahead of a general election.
He said: "Voters in the east, as in the rest of the UK, are concerned about open door migration, about pressure on green field land and about living standards - but they also wanted to give the other parties a bloody nose and we have to recognise Ukip was a convenient vehicle for that.
"We now need to convince people to stick with us.
"We need to build on our strategy, work on our vision and broaden our agenda.
"If we do that we can be confident of winning seats in Parliament."
Richard Howitt, who was re-elected as the Labour MEP in the East, said the party must acknowledge Ukip's success but not mirror their policies.
He said: "Labour has seen a big increase in its vote in this region but we must acknowledge the concerns that Ukip has tapped into - we do not reject those concerns but we do reject the prescription offered by Ukip. We say that Farage is a mirage."
Being pushed into second place in a national poll with just 12 months to go until the general election will add to questions about Mr Miliband's ability to take the keys to No 10.
Labour sources said although it looked as if Ukip was on course for victory, the vote for Mr Miliband's party was "up significantly" on the 15.7% it achieved in 2009.
Mr Clegg - who has acknowledged the Liberal Democrats could lose all of their 11 MEPs - is facing MPs' questions over his leadership and calls to quit from activists, including general election candidates.
Mr Cameron has been urged by influential Tory MP David Davis to bring forward his promised EU referendum by a year to 2016 to persuade defectors to Ukip that he is serious.
A senior Lib Dem source said it was "touch and go" whether the party would be wiped out in the European elections.
"It's not looking good," the source said, adding that it was "touch and go in one or two regions".