The defeat will encourage rebels within Brown's party urging a change in leadership before general elections that must be held before mid-2010.
Voters in the northwestern district of Crewe and Nantwich, who had elected the Labor candidate in every ballot since 1983, handed a landslide victory to Conservative candidate Edward Timpson, final results showed yesterday.
Cameron pronounced the result "the end of New Labor," the brand name for a one-time staunchly socialist party transformed by Brown and his predecessor, Tony Blair, into a centrist, free-market political machine.
"It's pretty bleak for Labor unless the economy picks up," said George Jones, professor emeritus of politics at the London School of Economics and Political Science.
"Global pressures are at work here, but people feel Brown is not governing with consistency and authority," Jones added.
Brown's initial burst of popularity after he succeeded Blair 11 months ago tempted him to consider an early national election - but the spectacle of him agonizing over the idea before finally scrapping it won him a reputation as a "ditherer."
The same charge stuck over the government's slow reaction to the near-collapse of mortgage lender Northern Rock, which was eventually nationalized.
Then came the government's decision to abolish a 10 percent income tax band intended to benefit low earners.
After protest from Labor lawmakers and pressure from Cameron, the government borrowed $5.3 billion to fund a relief package.
It all translated into a disastrous performance this week and in the May 1 municipal elections.
"This election probably marked the point where the government passed the point of no return," political analyst Anthony Howard said in a British Broadcasting Corp. television interview.
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