Prime Minister Tony Blair announces when he will step down on Thursday, 10 years after winning power in what was hailed as a new dawn for Britain that has since been darkened by the Iraq war.
Blair, U.S. President George W. Bush's closest ally over Iraq, leaves office out of favor among voters for sending British forces to join the 2003 U.S.-led invasion.
A ruling Labour Party rebellion in September forced him to say he would quit within a year to allow his long-time heir apparent, finance minister Gordon Brown, to take over.
But 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 it from its left-wing roots to the center of British politics.
An opinion poll published by the Guardian newspaper on Thursday showed 60% of voters believed Blair would be remembered as a force for change, though not always good. The ICBM poll said 44% 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 neighbor. 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 would make a statement about his future on Thursday. 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 Labor leader towards the end of June, triggering a leadership contest in which Brown is the only serious contender.
Under Britain's political set-up, as Labour leader Brown would also become prime minister and would not have to fight national elections 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 main opposition Conservatives in the opinion polls before the next elections.
Conservative leader David Cameron, 40, has revitalized the party of Margaret Thatcher -- the only prime minister to hold power longer than 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 national elections.
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 this.
British newspapers have questioned whether Brown can match Blair in the charm stakes, often portraying him as dour.