Nigel Farage says his party is a "serious player" for 2015 after UKIP made significant gains in the local elections, leaving the other main party leaders admitting there was work to do.
The Prime Minister conceded the Conservatives had to start delivering on immigration and welfare reform, and said the public had become "frustrated" with the status quo.
With just a handful of results still to be called, Mr Farage successfully forecast that his party would win double the 80 seats predicted.
Mr Farage told the leaders of the main parties he would "see them in Westminster" as the results showed UKIP now had the potential to cause an "earthquake" at next year's General Election.
He said: "The UKIP fox is in the Westminster henhouse."
Asked whether the party would widen its pitch to voters beyond the EU and immigration, Mr Farage insisted it already had "clear principles" on cutting taxes and bringing back grammar schools.
David Cameron said the Conservatives had got to work harder He added: "Don't think the immigration issue is going to go away. The plight of the eurozone is such that immigration is likely to be an even bigger question at the time of the general election."
With two results still to come, UKIP had gained 167 seats - giving them 184 seats in all but with no councils under their control; and Labour had won 3,999 seats, giving it control of 81 councils - an increase of five.
The Conservatives have 2,679 seats overall, controlling 41 councils - down 13; the Liberal Democrats picked up 882 seats , seeing the number of councils under their control falling from eight to six.
UKIP took seats off both Labour and the Conservatives in their heartlands, which was particularly damaging to Ed Miliband, who has been accused by his own party of running an "unforgivably unprofessional" campaign.
Ed Miliband celebrated in Redbridge but said discontent had built up Labour lost its grip in the north and, in a serious blow, lost control of Thurrock, a key marginal for the General Election in 2015.
Mr Miliband rejected that he was an electoral liability and told Sky News: "I think we ran a good campaign."
"I think in some parts of the country we have had discontent building up for decades about the way the country has been run and about the way our economy works … so what you are seeing in some parts of the country is people turning to UKIP as an expression of that discontent."
UKIP made its greatest gains in Essex, where Margaret Thatcher once identified the "Essex Man", a man who moved out of London, once voted Labour but switched to the Tories.
Deputy Prime Minister Nick Clegg admitted the Lib Dems had a bad night David Cameron said: "We have got to work harder and we have got to really deliver on issues that are frustrating people and frustrating me, like welfare reform and immigration and making sure people really benefit from this recovery."
Education Secretary Michael Gove told Sky News there were lessons to be learned from UKIP's success.
According to the latest Sky News projection, the results so far would give a hung parliament at the 2015 General Election, with Labour holding the largest number of MPs.
Sky's election analyst Professor Michael Thrasher said UKIP's success suggested the party would claim at least one seat in the House of Commons next year - most likely in Castle Point, Essex.
Sky's European elections coverage starts at 9pm on Sunday As predicted, the Liberal Democrats suffered significant losses, losing control in Kingston-Upon-Thames and in Portsmouth.
Liberal Democrat leader Nick Clegg admitted the party had had a bad night but said: "Actually I think in the areas where we have MPs where we have good organisation on the ground ... we are actually doing well."
Mr Farage celebrated by going on a tour of the areas where he had been most successful.
Siobhan Robbins, who was with him in Thurrock, said: "It was like he was a pop star. People were gathered around him and there was a scrum. I saw two grown men cry. One of them was saying: 'Thank you Nigel Farage. Thank you for showing us the light'."
:: Follow all the results as they come in on Twitter with @skyelections.
From left: Andrew Bridge, Steve Warren, James Male and Paul Goslin
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