May 9 (Bloomberg) -- Prime Minister Gordon Brown's Labour Party trailed the opposition Conservatives by 26 percentage points in a YouGov Plc opinion poll, the most the party has been behind since 1968.

The poll put Labour on 23 percent, down 3 points, the Conservatives on 49 percent, up 5 points, and the Liberal Democrats unchanged on 17 percent. Repeated at a general election, Conservative leader David Cameron would win the biggest Parliamentary majority of modern times.

Labour slumped to its worst performance since at least the early 1970s in local elections on May 1, the first ballot-box test since Brown took over from Tony Blair in June. Labour slipped to third place with 24 percent of the vote, and Conservative Boris Johnson ousted Labour's Ken Livingstone as London mayor.

``Too many voters felt our description of their lives wasn't what they experienced,'' Innovation and Skills Secretary John Denham told Labour supporters at a meeting in London yesterday evening. ``They won't forgive us for not understanding what's going on in their lives.''

YouGov said the Conservatives are enjoying a post-election boost that will probably fade. The party extended its lead over Labour from 18 points in a YouGov poll published on April 24.

`Halo Effect'

``We need to be cautious about any poll that shows large shifts of support,'' said Anthony Wells, an analyst at the polling firm. ``Big election victories like the local elections last week often have a halo effect. I doubt the Conservatives will stay at quite this level.''

Cameron said ``there is not one ounce of complacency'' in his party. In 2004, Blair suffered a backlash in local elections over the Iraq war, and led Labour to a third straight term in a general election the following year.

``We don't want to ride to victory on the back of an unpopular government, but to really convince people that we have got the right answers,'' Cameron told reporters in London after talks with New York City Mayor Michael Bloomberg. The mayor is founder and majority owner of Bloomberg LP, parent of Bloomberg News.

Brown blamed the election drubbing on rising food and fuel prices and global economic turbulence rather than his leadership, saying he is the best person to lead the country after presiding over a decade of expansion as finance minister.

Still, he acknowledged that public anger over a tax change that left 5.3 million of the poorest households worse off had played a role, and promised to compensate those affected.

Safe Seat at Risk

Brown now faces the loss of a safe seat in Parliament on May 22 at a by-election triggered by the death of Labour lawmaker Gwyneth Dunwoody last month.

William Hill Plc today made the Conservatives the clear favorite to win Crewe and Nantwich, a northwest England constituency held by Dunwoody since 1974. The bookmaker is offering odds of 2/5 on a Conservative victory, meaning a successful five-pound bet wins two pounds. It put Labour in second place on 9/4, followed by the Liberal Democrats on 10/1.

Dunwoody had a majority of 7,078 at the 2005 general election, and her daughter is the current Labour candidate.

Brown has until mid-2010 to hold the next general election and is counting on the economy recovering from next year to revive his political fortunes.

The Treasury forecasts economic growth will reach around 2.75 percent in 2010, compared with 2 percent this year and 2.5 percent in 2009. The International Monetary Fund predicts growth of 1.6 percent this year and next, the weakest since the end of the last recession in 1992.

The poll indicated that replacing Brown would damage rather than help Labour, with voters saying they would be less likely to back the party if it was led by someone else. Leadership candidates mentioned included Education Secretary Ed Balls, Foreign Secretary David Miliband, Health Secretary Alan Johnson and Blair.

YouGov interviewed 1,571 adults online May 7-8. No margin of error was given.

To contact the reporter on this story: Robert Hutton in London at rhutton1@bloomberg.net.

Last Updated: May 9, 2008 08:57 EDT