LONDON - A Conservative lawmaker with a knack for offensive remarks ousted the left-wing mayor of London in an upset that capped the ruling Labour Party's worst local election showing in four decades.

Results released early today showed Boris Johnson defeating Ken Livingstone in Labour's first test at the polls since then-Prime Minister Tony Blair handed the reins last year to Gordon Brown, who has since been dogged by accusations of indecision and incompetence.

Voters also picked opposition candidates in more than 300 municipal council races, prompting Brown to humbly pledge to heed the scathing verdict.

Conservative leader David Cameron said his party's strong gains represented a key moment on the path to ousting Brown at the next national election, to be held before mid-2010.

"Three years ago the idea that the Conservatives would win London and build up a 20-point lead across the county would have been literally unthinkable," Cameron said.

"I do hope that it does show that the Conservatives have changed into a party that can again be trusted," said Johnson, a former magazine editor, shortly after the result was announced to cheers from raucous supporters. "Let's get cracking tomorrow, and let's have a drink tonight."

Livingstone said the blame for his defeat must rest at his door, not Brown's.

"I accept that responsibility, and I regret that I couldn't take you to victory," the veteran politician said, looking pale and crestfallen. He leaves office immediately.