By Peter Griffiths
LONDON (Reuters) - The leader of Britain's rising right-wing anti-EU party on Friday promised a political "earthquake" with victory in next year's European elections, a challenge that could threaten David Cameron's hopes of a second term as prime minister.
Nigel Farage, head of the anti-mass immigration UK Independence Party (UKIP), said it would overturn decades of dominance by Britain's main three parties, the Conservatives, Labour and the Liberal Democrats.
Once derided by Cameron as "fruitcakes, loonies and closet racists", UKIP has increased its support to around 10 percent, according to pollsters YouGov, after taking just three percent of the vote in the last national election in 2010 and failing to secure a single parliamentary seat.
Farage said if UKIP wins a majority of the UK seats in the 2014 election for European Parliament it would effectively be a condemnation of "open-door immigration" and Britain's membership in the 28-nation bloc, which the main parties generally support.
UKIP had a 16.5 percent share of the vote at the last European elections in 2009, securing 13 of Britain's 72 seats.
"We can come first and cause an earthquake," Farage told his party's annual conference on its 20th anniversary. "We're changing the face of British politics."
Cameron has already promised to hold an "in/out referendum " before the end of 2017 in a move seen as an attempt to placate Conservative eurosceptics and take the advantage from UKIP. UKIP wants to leave the EU immediately.
Farage, 49, rejected the idea that withdrawal from the EU, Britain's biggest trading partner, would damage Britain's economy and global standing.
"There are those who say we can't go it alone. Those are the true voices of Little England," he told delegates.
UKIP secured nearly one in four of the votes cast at elections for local government jobs in May, buoyed by growing distrust of an EU many see as a threat to their sovereignty.
Surveys suggest it will do even better in the European elections next year. Under Britain's "winner takes all" system for national elections, it may still struggle to secure its first seat in the British parliament in the next vote in 2015, however.
The party's fast growth has opened it up to closer scrutiny that has led to accusations of racism, right-wing extremism and homophobia.
Its founder Alan Sked, who left the party in 1997, said last year that UKIP had become "extremely right-wing", "morally dodgy" on immigration and lacked intellectual seriousness.
Farage did nothing to dispel that image on Friday, saying immigrants to Britain had caused a crimewave, milked the welfare system and strained demand for housing, healthcare and schools.
"I shall get severely criticized for this, but I have to say that there is an even darker side to the opening of the door," he said. "London is already experiencing a Romanian crimewave."
A former commodities trader who cultivates an image as a man of the people, posing for pictures with a cigarette and a pint of beer, Farage denies being racist or extremist.
British Influence, a campaign group that wants to keep Britain in the EU, accused him of scaremongering.
"Nigel is a demagogue, whipping up fear and hatred with a smile on his face and a pint in his hand," said Peter Wilding, the group's director.
(Editing by Sonya Hepinstall)