* Coalition ally condemns Cameron referendum pledge
* Say "fiddling around" with EU treaty will hurt economy
* Polls show Conservative bounce at expense of anti-EU party
By Peter Griffiths
LONDON, Jan 27 (Reuters) - Nick Clegg, leader of the junior party in Britain's ruling coalition, denounced David Cameron's pledge to hold a referendum on quitting the European Union, as polls on Sunday indicated the prime minister's move may gain him votes.
"It is not in the national interest when we have this fragile recovery," said Clegg, whose Liberal Democrats strongly favour closer EU ties, in contrast to many members of Cameron's Conservative party. "I don't think it helps at all."
He dismissed as "implausible" Cameron's plan to take back powers from Brussels before a referendum on a new treaty by 2017 that would let voters take Britain out. EU leaders have shown little wish to grant Cameron concessions and Clegg said EU talks would distract ministers from efforts to revive the economy.
Cameron, he told the BBC, would damage economic growth if he spent "years flying around from one European capital to the next, fiddling around with the terms of Britain's membership".
The LibDems are languishing in the polls and are unlikely to leave the coalition before an election in 2015, but the EU issue has added to strains. Cameron, who says he wants Britain to stay in the EU, last week promised a referendum if he is re-elected. It is less clear what may happen if treaties remain unchanged.
The first opinion polls published since he made his pledge of an "in-out" vote, however, showed that the prime minister may be succeeding in reversing a drift from the Conservatives to a party which campaigns for Britain to leave the European Union.
A Survation poll in the Mail on Sunday, which showed Labour unchanged and in the lead on 38 percent, put the Conservatives on 31 percent, up two points, while the UK Independence Party was down by the same margin, on 14 percent. UKIP's surge from just 3 percent in the 2010 election has raised the prospect of a split on the right that could condemn Cameron to defeat.
Another poll, by ComRes in the Independent on Sunday, showed an even more marked "Brussels bounce" for the prime minister, with the Conservatives gaining five points from last month to 33 percent and UKIP losing four points to be on 10 percent. Again, ComRes put Labour in the lead, down a point on 39 percent.
Cameron's European move worries the United States and EU allies, which want Britain to stay in the bloc. Many business leaders say it creates dangerous uncertainty.
Many Conservatives, whose party toppled previous premiers over European policy, welcomed a referendum after 2015. However, without improvement in an economy which shrank by 0.3 percent in the last quarter, Cameron's re-election is far from certain. (Editing by Alastair Macdonald)