Nigel Farage has claimed his party is a "serious player" for 2015 after UKIP made significant gains in the local elections, leaving main party leaders admitting the public was voicing its discontent.
The Prime Minister conceded his party had to start delivering on immigration and welfare reform, and said the public had become "frustrated" with the status quo.
Mr Farage forecast his party would win double the 80 seats predicted leading to claims the elections had ushered in an era of four-party politics.
A ballot box is emptied at Trinity School in Croydon
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.
David Cameron said: "People want us to deliver. The economy is growing, we are creating jobs, but 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.
"We will be working flat out to demonstrate that we do have the answers to help hard working people."
Labour lost its grip in the north and, in a serious blow, the party lost control of Thurrock, a key marginal for the General Election in 2015, to no overall control, losing two seats to UKIP.
Mr Farage said UKIP would now be "serious players" in the General Election and said the party was expecting to win double the 80 seats that had been predicted.
He said: "The UKIP fox is in the Westminster henhouse" and added: "The idea the UKIP vote just hurts the Tories is going to be blown away by this election."
Joey Essex may have supported Ed Miliband but Essex man voted UKIP
However, Mr Farage, who has consistently warned the local and European elections would deliver a UKIP "earthquake", admitted that the party was unlikely to be successful in London.
Ed Miliband defended the party's campaign and said people were turning to UKIP to express their discontent with the way the country is run.
He told Sky News: "I think we ran a good campaign."
He said: "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 and people feeling that the country just does not work for them and so what you are seeing in some parts of the country is people turning to UKIP as an expression of that discontent and that desire for change."
Mr Farage casts his vote in Cudham, Kent
Mr Miliband added that Labour would be able to provide the answer to the discontent by the 2015 vote.
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.
UKIP took seats from Labour in Hartlepool, won 10 seats in Rotherham and polled more than a third of the vote in wards in big cities, such as Sunderland, Birmingham and Hull, where it previously had little or no presence.
The Camerons and the Milibands cast their votes
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
As predicted, the Liberal Democrats suffered significant losses, particularly in Kingston-Upon-Thames where it lost control of the council after 12 years to its coaltion partners. In Portsmouth it lost control with UKIP gaining six seats.
However, the party managed to hold on in Eastleigh, where UKIP came second last year in a by-election.
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.
Sky's Siobhan Robbins, who followed from Thurrock in Essex to Basildon, said that in Thurrock he had been greeted by a large crowd.
She 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'."
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