By Andrew Osborn
LONDON (Reuters) - The anti-European Union UK Independence Party made big gains in local elections, siphoning support from Prime Minister David Cameron's Conservatives in a vote that underlined the threat it poses to his re-election chances in 2015.
Early results showed UKIP had won 42 council seats - as many as Labour - after seven of 35 councils had been declared and that it had polled an average 26 percent of the vote, the best result by a fourth party since World War Two.
UKIP, which wants Britain to leave the European Union and an end to "open-door immigration", also pushed Cameron's Conservatives into third place in an election for a national parliamentary seat in northern England, a humiliating blow for the prime minister.
The results showed UKIP could split the centre-right vote at the next national election, making it harder for Cameron to defeat Labour, which leads his Conservatives by up to 10 percent in opinion polls as economic austerity persists.
Friday's outcome is also likely to reignite questions about Cameron's leadership from malcontents within his own party who complain he is too liberal and to pile pressure on him to take a tougher stance on Europe and immigration, issues on which he has already tacked to the right.
Full results of the elections for more than 2,000 council seats in England and Wales are expected later on Friday.
UKIP said it had tapped a wider public disenchantment with Britain's three mainstream parties, which it argues are effectively all left-leaning social democrat-style parties.
"We've got three parties who have given away the ability to govern our own country, who have led us into near bankruptcy and who have pursued open door immigration policies," Nigel Farage, UKIP's leader, told BBC radio.
"We want to fundamentally change British politics. It can happen."
Though represented in the European Parliament, UKIP currently has no MPs in the British parliament.
Labour, which has controlled South Shields since 1935, held onto the national parliamentary seat that was previously occupied by David Miliband, brother of Labour leader Ed Miliband and a former foreign minister. But UKIP won 24 percent of the vote, its second highest result in such an election.
Grant Shapps, the chairman of the Conservative party, said his party, the senior partner in a two-party coalition, had heard the voters' message "loud and clear".
"We are offering a lot of the things that people say they're concerned about," he said.
But he emphasized the local nature of the election. "People's aren't voting for who runs the country, they're voting for local councils." The real choice at the next national vote in 2015 would be between Labour and the Conservatives, he said.
He and politicians from other parties have said UKIP's growing 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.
Cameron has called the party "fruitcakes and closet racists" and several of its candidates were suspended from running in the elections after it emerged they had once belonged to far-right groups.
(Additional reporting by Peter Griffiths; Editing by Guy Faulconbridge and Jon Boyle)