British Prime Minister Tony Blair said on Thursday he will resign on June 27, 10 years after coming to power in what was hailed as a new dawn for a Britain that has since been darkened by the Iraq war.

Blair's resignation triggers a contest for the leadership of the ruling Labor Party, which Finance Minister Gordon Brown is favorite to win. Brown would then become prime minister.

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"I've been prime minister of this country for just over 10 years," Blair told party members in Trimdon in his northern England constituency.

"I think that's long enough, not only for me, but also for the country and sometimes the only way you conquer the pull of power is to set it down."

The closest ally of U.S. President George W. Bush in Iraq leaves office out of favor among voters for sending British forces to join the 2003 U.S.-led invasion.

A Labor Party rebellion in September forced him to say he would quit within a year to allow Brown, his long-time heir apparent, to take over.

But Blair will also be remembered for helping bring peace to Northern Ireland after decades of violence, winning three straight elections for Labor for the first time and dragging it from its left-wing roots to the centre of British politics.

An opinion poll published by The Guardian newspaper on Thursday showed 60 percent of voters believed Blair would be remembered as a force for change, though not always good. The poll said 44 percent believed he had been good for Britain.

Blair had long been expected to hand over power before the end of his third term to let another Labour leader guide the party into the next national elections, expected in 2009.

Brown, whose official residence is next door to Blair's in London's Downing Street, has waited with increasing impatience for the departure of his neighbour. Critics say their rivalry, often bitter, has diluted the government's effectiveness.

Blair quits as only the second prime minister in a century to have served 10 years, tainted by a corruption scandal in which he became the first serving prime minister to be quizzed by police in a criminal probe.

Brown's chief challenge will be to revive support for Labor and overtake the opposition Conservatives in the opinion polls. Conservative leader David Cameron, 40, revitalized Margaret Thatcher's party and was the only prime minister to hold power longer than Blair in the past century since becoming leader in 2005. Polls suggest he could win a slim majority in parliament in national elections.