Britain's Blair to announce departure today

LONDON, May 10 (Reuters) Ten years after sweeping into power in Britain, an increasingly unpopular Tony Blair will announce today when he plans to step aside and allow finance minister Gordon Brown to take over as prime minister.

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

But he leaves office out of favour among voters and within his party for sending British troops to Iraq in 2003. A Labour rebellion in September forced him to say he would quit within a year.

He had long been expected to hand over power before the end of this third term to let another Labour leader pilot the party into the next election, 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.

His spokesman said Blair will make a statement about his future today. The prime minister will attend a cabinet meeting in London and then fly to his constituency in northern England to speak to local supporters.

Blair is expected to announce he will quit as Labour leader towards the end of June, triggering a leadership contest in which Brown is the only serious contender.

Under Britain's constitution, as Labour leader Brown would also become prime minister and would not have to fight a general election until May 2010 at the latest.

Blair and Brown were the twin architects of Labour's rise to power in 1997 after 18 years in the political wilderness and Britain's long-serving finance minister is now certain to finally get the job he has coveted for so long.

His chief challenge will be to revive support for Labour and overhaul the opposition Conservatives in the opinion polls before the next election.

Conservative leader David Cameron, 40, has revitalised the party of Margaret Thatcher -- the only premier to outlast Blair in the past century -- and Winston Churchill since he became leader in 2005, and polls suggest he could win a slim majority in parliament in a national election.

Brown is widely respected for presiding over a decade of strong economic growth and for granting independence to Britain's central bank to set interest rates.

He has also overseen huge increases in spending on education and health. But Labour has not been rewarded by the electorate for its public service spending and Brown does not have the charismatic charm that helped Blair rule for a decade.

Reuters JK SBA VP0422