Conservatives have overtaken the Labour opposition in an opinion poll for the first time this year, enjoying a bounce on the back of Prime Minister David Cameron's veto of a new European Union treaty, the latest Reuters/Ipsos MORI poll showed on Wednesday.

The rise in support for Cameron's Conservatives is all the more remarkable given Britons' increasing pessimism on the economy, with only 12 percent expecting it to improve in the next year, the lowest figure since the credit crunch began to bite in September 2008.

Support for the Conservatives rose by seven percentage points to 41 percent, while backing for centre-left Labour slipped two points to 39 percent.

The trend was confirmed by a separate survey by ComRes for the Independent newspaper published on Wednesday that put the two largest parties neck-and-neck on 38 percent.

The polls could worry Labour leader Ed Miliband, whose party is defending a parliamentary seat in a by-election in a West London suburban constituency on Thursday.

A national election is not due until 2015 and the Conservative-led coalition has vowed to serve until then to try to break the back of a big budget deficit.

However, continued buoyancy in the polls might tempt some Conservative lawmakers to press for an early election to try to secure an outright majority.

The Liberal Democrats, the junior partners in the coalition that took power in May 2010, were on 11 percent in the Ipsos MORI poll, down one point at less than half what they polled in the last election 18 months ago.

Ipsos MORI spoke to around 1,000 Britons on December 10-12, after Cameron's historic use of his veto last week prevented the EU from pushing ahead with a new treaty to tackle the euro zone crisis.

Cameron says he acted after failing to achieve safeguards he was seeking for Britain's financial services industry.

Support for Cameron and Chancellor George Osborne's response to the euro zone crisis has gone up four percentage points since November.

Fifty-six percent said they had responded very or fairly well to the crisis, while only 40 percent said European politicians such as French President Nicolas Sarkozy and German Chancellor Angela Merkel had responded well.

BOUNCE TO FADE?

The EU summit's fallout has overshadowed Osborne's autumn statement last month, when he was forced to cut economic growth forecasts and announced that austerity measures would continue until 2017, two years after the next election must be held.

Commentators said they expected the Conservative bounce to be fairly short-lived.

The surprise for me would be if it lasted longer than two months, said Justin Fisher, politics professor at Brunel University.

Cameron has been very good at selling his veto as something that is good for Britain, he added.

Fisher said the poll showed the vulnerability of opposition Labour, struggling to regroup under the leadership of Miliband, and the Lib Dems, for whom governing in a coalition has been a sobering experience after decades in the political wilderness.

Liberal Democrat leader Nick Clegg criticised the outcome of the EU summit as bad for Britain but many Britons are sceptical about the benefits of closer European integration and much of the press is openly hostile to the EU.

The slump in support for the Liberal Democrats means Clegg has little choice but to remain in the coalition rather than risking an election drubbing.

Seventy percent of those polled identified the state of other countries' economies as the greatest threat to Britain's national interests.

Chancellor Osborne has said that a recession in the euro zone would be likely to send the economy into reverse. Britain does around half of its trade with the EU.

Sixty percent of Britons said the economy would get worse next year, against the 12 percent who saw an improvement. Unemployment is expected to rise over the next year and the economy will grow only slowly, at best.

Britons were divided on whether the government had made the right decisions on cutting the budget deficit. Forty-six percent said it had made the wrong decisions, while 44 percent backed tough cuts in public spending.

* Ipsos MORI interviewed 1,001 adults across Britain between December 10 and 12 by telephone; data are weighted to the profile of the population.

(Reporting by Keith Weir; Editing by Kevin Liffey.)