By Andrew Osborn
LONDON (Reuters) - Britain's anti-EU UK Independence Party said on Sunday it would use its growing success to try to secure an early referendum on leaving the European Union, after its support hit a record high of 25 percent in an opinion poll.
The poll, published days after UKIP won its first elected seat in Britain's parliament at the expense of Prime Minister David Cameron's Conservative Party, suggested it could pick up more seats than previously thought in a national election in May.
UKIP favours a British exit from the European Union, known as a 'Brexit', and tighter immigration controls. It has shaken up the British political landscape, challenging its traditional two-party system and piling pressure on Cameron to tack further to the right.
UKIP leader Nigel Farage said he would demand that Cameron bring forward a planned referendum on EU membership from 2017 to next year if UKIP polled strongly and the prime minister needed its support to stay in office.
"I'm not prepared to wait for three years. I want us to have a referendum on this great question next year and if UKIP can maintain its momentum and get enough seats in Westminster we might just be able to achieve that," Farage told the BBC.
UKIP's rise threatens Cameron's re-election drive by splitting the right-wing vote, increases the likelihood of another coalition government, and poses a challenge to the left-leaning opposition Labour party in northern England too.
It also adds to pressure on Cameron from within parts of his own party to become more Eurosceptic.
Cameron has promised to try to renegotiate Britain's EU relations if re-elected next year, before offering Britons a membership referendum in 2017. But some of his own lawmakers want him to take a tougher line and to bring forward the vote.
UKIP won European elections in Britain in May, has poached two of Cameron's lawmakers since late August, and will try to win a second seat in parliament in a by-election next month.
Before Sunday, most polling experts had forecast it could win only a handful of the 650 seats in parliament in 2015.
But based on the result of a Survation poll for The Mail on Sunday, the party could win more than 100 seats in 2015.
The poll put UKIP's support at 25 percent, 2 percent higher than a similar poll in September.
Support for the Conservatives and Labour was tied at 31 percent, according to the poll, which was based on interviews with 1,003 people nationwide.
Given the country's winner-takes-all electoral system, a three-figure seat count for UKIP seems unlikely.
The party itself is aiming for between 12 and 25 seats. Two other polls on Sunday put UKIP's support at 16 and 17 percent.
PRESSURE ON CAMERON, MILIBAND
But all three polls underline the party's rise.
In 2010, the last time a national election was held, UKIP won just 3.1 percent of the vote and no seats in parliament.
Labour leader Ed Miliband, whose party came close to losing a safe seat to UKIP on Thursday, wrote in The Observer newspaper that he recognised UKIP was "tapping into a seam of discontent and despair that Labour cannot – and will not – ignore."
He said he would not react with a knee-jerk policy change but did not set out specific new policies in response.
UKIP's success has piled pressure on Miliband, who is already facing questions from some of his own lawmakers about his image and performance, to pay more attention to the concerns of voters outside London.
Harriet Harman, the party's deputy leader, shrugged off such criticism. "We are not going to have a wobble or a leadership change," she told the BBC on Sunday.
UKIP's rise has created serious problems for Cameron too.
The Mail on Sunday quoted an unnamed cabinet minister as saying Cameron would face a leadership challenge if his party failed to defeat UKIP in a by-election in Rochester, southern England, as expected next month.
"Cameron would win. But it would be very damaging," the minister was quoted as saying.
UKIP's Farage predicted that a victory for his party in Rochester could unseat Miliband or Cameron.
"There is a possibility that one or both of those leaders may not be leading their parties into the next general election," he said.
(Editing by Aidan Martindale and Jon Boyle)