LONDON (Reuters) - Nearly nine in 10 of the over four million Britons who backed the anti-EU UKIP party in this month's European elections will vote for them again in national polls next year, potentially costing Prime Minister David Cameron vital votes, according to a survey on Saturday.
If that were to happen, it would break the previous trend of support for UKIP in EU ballots disappearing at parliamentary elections.
UKIP, which calls for Britain to leave the European Union, won 4.3 million votes and topped the vote last week, scoring a 27.5 percent share, ahead of the main opposition Labour Party and ruling Conservatives.
However, in the 2009 European elections, the party took 16.5 percent of the vote before falling to just 3 percent a year later in a general election, failing to win a single seat in parliament.
Britons are expected to elect a new government by May 2015, with Cameron - currently at the head of a coalition with the junior Liberal Democrat party - promising to renegotiate Britain's EU membership terms and hold an in/out referendum within two years, if his party achieves an overall majority.
Saturday's ComRes poll, however, suggests that his chances of winning the 2015 election could be harmed, with over 3 million voters intending to stick with UKIP.
The poll found that the largest share of those who backed UKIP and its leader Nigel Farage had supported the Conservatives in the 2010 general election.
Just under 40 percent of those who voted for UKIP last week said they were "certain" to back the party next year and another 49 percent said they were "likely" to do so, according to the poll funded by UKIP donor Paul Sykes and published in the centre-right Telegraph newspaper.
That may not necessarily bring UKIP its first parliamentary seat however.
European polls use a system based on proportional representation, whereas parliamentary elections are decided by a first-past-the-post system seen as favoring the larger established parties.
UKIP is hoping to win its first Member of Parliament (MP) at a single-seat election on Thursday in the central England constituency of Newark, currently held by the Conservatives with a large majority.
ComRes interviewed 4,078 adults between May 23 and 26 for the poll.
Reporting By Costas Pitas; editing by Stephen Addison