While Blair, his reputation as a reformer tainted by the Iraq war, was expected to become a Middle East peace envoy, Brown highlighted his determination to stamp his own authority after being asked by Queen Elizabeth II to form a new government.

Brown pledged in his first public comments to lead a "new government with new priorities."

He said he wanted to "build trust" in the government and reject "old politics" as he showed his determination to make a break with the Blair years. "Now let the work of change begin," he declared outside his Downing Street office.

The handover came shortly after Blair was given a standing ovation in the House of Commons at his last appearance before heading off to tender his resignation to the monarch.

"That is that. The end," Blair said, choking back emotion, as he concluded his last question-and-answer session and listened to many tributes to his 10 years in office marked by a booming economy as well as the 2003 US-led invasion of Iraq.

Even as he bowed out Blair fuelled speculation that he will become an envoy for the Middle East Quartet -- the United States, Russia, the European Union and United Nations -- saying a two-state solution is the only way to end the conflict between the Israelis and Palestinians.

"I believe it is possible to do that but it will require a huge intensity of focus and work," added Blair, who had Brown, a political ally turned rival, sat at his side.

Blair told lawmakers he was "sorry" for the danger faced by soldiers fighting in Iraq and Afghanistan, two of the most controversial decisions of his premiership.

But he was defiant, refusing to accept critics' views that British troops are fighting in vain.

"I don't and I never will. I believe they're fighting for the security of this country and wider world against people who would destroy our way of life," he said.

Most of the session was taken up with plaudits from lawmakers from his own Labour party and Conservatives and Northern Ireland First Minister Ian Paisley, whose recent election climaxed one of Blair's biggest achievements.

Paisley wished Blair well in any Middle East role, drawing a parallel with the reconciliation between Protestants and Roman Catholics in Northern Ireland.

"I hope that what happened in Northern Ireland will be repeated," he said, calling it a "colossal task" for Blair.

Blair paid tribute to lawmakers who have assailed him at the weekly prime minister's questions over the last decade.

"Some may belittle politics but we know who are engaged in it that it is where people stand and fall," he said. "If it is on occasion the place of low skullduggery it is more often the place for the pursuit of noble causes."

The 56-year-old Brown, whose formal title is Chancellor of Exchequer, took over from Blair as Labour Party leader on Sunday.

He must call an election by May, 2010 and has vowed to learn "the lessons" from the Iraq war. Brown received an early political boost Tuesday when a lawmaker from the opposition Conservative Party defected to join Labour.

Brown stewarded Britain through record economic expansion but his serious style is a marked contrast to Blair's eloquent and easy, media-friendly persona.

He is widely seen as more sceptical of integration with the European Union and more cautious in his relations with Washington.

Brown has promised to stay true to Blair's progressive centre-left agenda but introduce a more open form of government with parliament at the centre.

Blair became prime minister in 1997 after 18 years of Conservative government and after leading Labour to the biggest parliamentary majority for half a century with a strong public mandate for change.

But his popularity ratings dropped considerably, mostly because of his decision to join the invasion of Iraq and support President George W. Bush's so-called "war on terror".

In an interview with The Sun newspaper published Wednesday, the US president rejected suggestions thatthat Blair was his uncritical "poodle", dismissing it as "just silly ridicule".

Blair has won praise at home for social reforms like gay rights and introducing a minimum wage, as well as resolving the sectarian conflict in Northern Ireland, and abroad for leading efforts to tackle climate change and increase aid and assistance to Africa.

Blair and Brown entered parliament in 1983, once shared an office and were the chief architects behind the restyling of their party into "New Labour", but their friendship soured as Brown believed Blair had reneged on a deal to hand over power sooner.

Brown was expected to announce his senior ministers Wednesday or Thursday.