LONDON (Reuters) - Britain's Eurosceptic UKIP party has made its strongest ever gains in local elections, harnessing discontent with immigration and established politicians to grab support from Prime Minister David Cameron's Conservatives and the opposition Labour party.
The surge by the UK Independence Party, which wants Britain to leave the EU, will pile pressure on Cameron to toughen his stance on Europe and alarm some Conservatives who worry UKIP could scupper their hopes of winning the 2015 national election.
If the trend indicated by partial results is mirrored in elections to the European Parliament, also held on Thursday in Britain, the votes will mark the biggest electoral triumph to date for UKIP's leader, Nigel Farage.
A former commodities trader who often poses with a pint of beer in his hand and a cigarette in his mouth, Farage styles himself as the antidote to Britain's established politicians whom he accuses of ignoring voter concerns about everything from immigration and the EU to the price of alcohol and crime.
Farage said the results felt as if the "UKIP fox was in the Westminster hen house" and promised to go all out to try to win seats in the British parliament in next year's national election, a target polls have indicated will be tough.
In an interview with Reuters, he predicted his success would force Cameron to become more Eurosceptic to stave off a leadership challenge.
"Really good solid performance from UKIP and right across the country. That's the really interesting thing: in big Tory areas, in big Labour areas we are scoring consistently up in the high 20 percents," Farage said on national television.
On a walkaround in Essex, a traditional Conservative heartland in southeast England where late Conservative leader Margaret Thatcher was once popular, Farage was greeted as a hero before heading for a pub where he pulled his own pint of beer.
"The whole country has had enough of the political elite running it and it is a much needed change," John Allen, a 41-year-old taxi driver who voted UKIP, told Reuters in Thurrock in Essex. "I have just had enough (of the main parties). They all sing from the same hymn sheet."
Partial results from over 90 percent of English councils showed UKIP won 155 new seats. Labour won 292 new seats, the Conservatives lost 201 and the Liberal Democrats lost 284.
Cameron once called UKIP a party of "fruitcakes, loonies and closet racists" and some of the party's members have been scolded for a series of racist, sexist and homophobic statements including advising women to clean homes properly and a claim that flooding was punishment for gay marriage.
Farage, who has denied his party is racist or sexist, had promised to use the European and local elections to trigger a "political earthquake" in Britain.
However, a different voting system in next year's national election which favours bigger parties will make that hard to achieve and opinion polls show many voters will revert to voting for the parties they have favoured traditionally.
A YouGov poll on voting intentions for next year's vote published on Friday had UKIP's support on 14 percent, behind the Conservatives and Labour who were tied on 34 percent each, underlining the scale of the challenge.
While Labour gained seats and looks on course to win the local elections, the margin of its probable victory is not as emphatic as its strategists hoped.
It had hoped to rack up bigger gains and as local elections are often used to express discontent with the incumbent government, Labour's showing stirred doubts internally about its ability to win a national election next year and prompted some to question the performance of its leader Ed Miliband.
UKIP made inroads into traditional Labour strongholds such as Rotherham in the north.
Both Miliband and Liberal Democrat leader Nick Clegg admitted the results showed a dissatisfaction with the established parties.
"There's a very strong anti-politics mood around, a restlessness and dissatisfaction with all the main parties," Deputy Prime Minister Clegg, who shares power with Cameron, said. He said he would not resign as party leader.
The European elections were held on the same day, but the results will not be announced until Sunday evening, in line with the rest of the EU. They will determine the political persuasion of Britain's 73 lawmakers in the 751-seat European Parliament.
Cameron has promised to try to reshape Britain's ties with the EU if re-elected next year and to give Britons an in/out EU membership referendum by the end of 2017. UKIP wants an immediate referendum.
UKIP has floated the idea of forming electoral alliances with Conservative candidates in next year's national election but Cameron on Friday rejected that idea.
Cameron said the results showed voters wanted him to deliver more in core policy areas.
"There was a clear message from last night's elections: people want us to deliver more on issues that frustrate them and frustrate me," he said. "We will work flat out to deliver more on the economy, immigration and welfare."