By Andrew Osborn and Guy Faulconbridge
LONDON (Reuters) - The anti-European Union UK Independence Party made sweeping gains in local elections, siphoning support from British Prime Minister David Cameron's Conservatives in a vote that exposed a threat to his re-election chances in 2015.
UKIP, in results released on Friday, secured almost one in every four votes cast on Thursday in council elections in mostly English rural areas that have traditionally been Conservative strongholds, rattling all Britain's three main parties as voters switched to the populist group.
UKIP, which wants Britain to leave the European Union and an end to "open-door immigration", said more than 1 million people had voted to endorse its policies, the best result for a party outside the big three since World War Two.
It also pushed Cameron's Conservatives into third place in an election for a national parliamentary seat in a traditional Labour stronghold in northern England, where Labour's majority was almost halved compared to three years ago.
"It sends a shockwave, I think, through the establishment," Nigel Farage, the former commodity trader turned politician who leads UKIP, said on Friday. "Today is a game-changer."
Cameron once dismissed UKIP as "a bunch of fruitcakes, loonies and closet racists", and a senior Conservative minister called the party "a collection of clowns" before the vote.
"Send in the clowns," Farage quipped to Sky News. "We have been abused by everybody, attacked by the entire establishment who did their best to stop ordinary decent people from going out and voting UKIP and they have done in big, big numbers."
Speaking after the vote, Cameron changed his tone, saying it was "no good" to insult a party that people had voted for and had contributed to his own party losing 335 council seats.
"We need to show respect for people who've taken the choice to support this party and we're going to work really hard to win them back," he said.
Though UKIP still has no seats in the 650-seat lower house of the British parliament, its surge in the polls will increase pressure on Cameron from nervous lawmakers in his own party to take a tougher stance on Europe and immigration.
The results suggest UKIP could split the center-right vote at the next national election. That would make it harder for Cameron to defeat Labour, which leads his Conservatives by up to 10 percent in opinion polls as the economy shows tentative signs of recovering despite unpopular public spending cuts.
While the results are likely to reignite questions about Cameron's leadership from malcontents who complain he is too liberal, they will also concern Labour leader Ed Miliband, who faces growing UKIP support in traditional Labour strongholds.
Though Labour, which has controlled South Shields since 1935, held onto the parliamentary seat, UKIP won 24 percent of the vote there, its second highest result in such an election.
Labour won 291 council seats, but that was not as many as expected, and Ed Miliband, its leader, said the results showed his party had more work to do to persuade doubters.
"The biggest opponent we will face at the next election is the idea that no one has any real answers and there is nothing we can do to turn things around," he said.
The Lib Dems, the junior partner in Cameron's coalition, had a bad election. They won so few votes in South Shields that they lost their deposit. They also lost 123 council seats.
Nick Clegg, their leader, said he could understand why voters were drawn to what he called UKIP's "simple answers", but said he did not think it had the solutions to "complex problems".
"The next general election will be all about which parties can govern this party in Westminster," he said.
UKIP won 147 council seats, up from eight in 2009. More than 2,000 council seats in England and Wales were contested.
Even though the Conservatives suffered heavy losses, they still won more seats - 1,116 - than any other party since many of the areas have long been Conservative strongholds.
Critics say UKIP's success means it will come under increasing scrutiny and that voters will ultimately find it does not have properly-developed policies, a claim UKIP rejects.
Farage says Britain's political leaders have failed voters by allowing far too many immigrants into the country and by allowing the European Union to purloin too much sovereignty.
Though significant, UKIP's gains are unlikely to give it control of a single council and Britain's first past the post system means it will have to poll even greater numbers if it is to win a large number of seats in a general election.
"None of us knows whether UKIP is a flash in the pan," Tony Travers, director of British government at the London School of Economics, said.
Farage told BBC TV his party was here to stay: "We've taken our first substantive steps to being a credible party that can win seats at Westminster."
(Additional reporting by Peter Griffiths, Stephen Addison and William James; Editing by Michael Roddy)