By Kylie MacLellan
BASILDON, England (Reuters) - British Prime Minister David Cameron will be forced to step up his drive to claw back powers from the European Union to head off a challenge to his leadership after losing votes to the anti-EU UK Independence Party, its leader Nigel Farage said.
Cameron has promised to give Britons a vote on EU membership by the end of 2017 if re-elected in a national vote next year, in part because of the dramatic rise of Farage's UKIP party which has transformed British politics by tapping into popular discontent about immigration levels and established politicians.
UKIP wants Britain to leave the EU immediately, arguing it will be more democratic and prosperous outside the bloc and be able to stem what it regards as an unsustainable flow of immigrants from EU states such as Romania and Bulgaria.
Farage, whose party is likely to have made significant gains in European elections after a strong result in a local vote, said UKIP's electoral success meant Cameron would now have to become even tougher on Europe to counter a probable challenge from inside his own Conservative party.
"Cameron will change policy," Farage told Reuters over coffee in the Castlemayne pub in Basildon, a town 25 miles east of London where late Conservative leader Margaret Thatcher remains popular.
"He will try and sound more Eurosceptic. I think he will try and pen in a bit the renegotiation time for our deal with Brussels because I think actually it is the renegotiation which weakens his argument," Farage said.
Cameron's Conservative party has long been convulsed by a strongly Eurosceptic wing within it. Partisan fights over Europe were the undoing of the last two Conservative prime ministers, John Major and Thatcher.
Cameron has promised to try to claw back a range of powers if re-elected next year to appease Eurosceptics and to then give Britons a referendum on whether to remain inside the EU by the end of 2017.
But he has so far garnered only limited backing for his plans among EU states and German Chancellor Angela Merkel has ruled out the prospect of a far-reaching overhaul of the bloc's treaties.
Farage, who styles himself as the antidote to Britain's established politicians, argues that Cameron will get nowhere with his renegotiation and that Britain will end up outside the European Union.
He said he would use the European elections to trigger a "political earthquake" in Britain, whose elite he accuses of ignoring voter concerns about everything from Europe to immigration.
"There are certainly a few tremors knocking around," Farage said, refusing to drink beer, his favored beverage, after earlier downing several pints of it in front of the cameras to celebrate the best election result in UKIP's 20-year history.
"We are going to find out Sunday night. I would say the signs look encouraging but I'm not going to count any chickens before they have hatched."
The 50-year-old former metals trader in the City of London has seen UKIP support soar in the eight years since Cameron dismissed it as a bunch of "fruitcakes and loonies and closet racists".
In English local elections on Thursday, UKIP poached seats from Conservative strongholds such as Basildon in the south and from Labour heartlands in the north such as Sunderland.
In Basildon, where Farage was greeted as a hero by local supporters on Thursday, UKIP won 11 seats to become the second largest party on the council. The Conservatives lost 7 seats but remain the biggest party in the area.
All three mainstream party leaders - Cameron, Labour leader Ed Miliband and Liberal Democrat leader Nick Clegg - admitted the vote showed disaffection with politicians and concern over the economy, welfare and immigration.
Farage said all three would have to change tack on Europe or find new jobs: "They will move their rhetorics, they will challenge their leaders' positions," he said.
"Miliband will be pressured to offer a referendum; Clegg will be pressured to become the next European Commissioner in June; and Cameron may find 46 signatures by the autumn to challenge his leadership," Farage said.
But if Farage secures a big win in the European elections, what will he do with his influence?
Farage said he would wanted Martin Schulz, a strong federalist, to lead the next European Commission because he felt Schulz's pro-European views were so abhorrent that they would undermine the European project he despises.
"I think Schulz would represent the true face of the direction the European union is going in. I think the British people will recoil in horror and we would win a referendum. I'm batting for Schulz."
Two sources said on Friday that Cameron was strongly opposed to Schulz getting the top job, precisely because he felt the German was too federalist.
Farage was less enthusiastic about Marine Le Pen's National Front in France, ruling out an alliance in the European Parliament with it.
"I don't want to be rude about Marine Le Pen, there is no need to do that, but I don't believe her party are part of our political family," he said.
Asked whether he could envisage working with her, he was blunt. "The answer is no," he said.
(Writing by Guy Faulconbridge and Andrew Osborn; editing by Philippa Fletcher)