LONDON — Prime Minister Blair, one of Britain’s most influential and long-serving leaders in a century, announced yesterday that he would step down on June 27, leaving behind a legacy of economic and political achievement mixed with deep public anger over his partnership with President Bush in the Iraq War.

“I have been prime minister of this country for just over 10 years,” Mr. Blair told cheering supporters at the Trimdon Labor Club in his home constituency of Sedgefield, speaking in the modest building where he launched his political career as a 30-year-old lawyer almost 24 years ago to the day. “In this job, in the world today, I think that is long enough for me, but more especially for the country. Sometimes the only way you conquer the pull of power is to set it down.”

Mr. Blair’s long-anticipated announcement clears the way for his political partner and rival Gordon Brown, the chancellor of the exchequer, to replace Mr. Blair as prime minister. Mr. Brown, the country’s successful and longest-serving finance minister, is expected to easily win a party leadership battle in the coming weeks then assume a premiership he has coveted for a decade.

In a sometimes emotional 15-minute speech, Mr. Blair expressed pride in his domestic efforts to shore up Britain’s economy and health care systems, reduce crime and unemployment, and guide Britain from the post-Cold War doldrums into the 21st century. He also defended his strong alliance with the Bush administration in Afghanistan and Iraq, saying he still believed it was right to “stand shoulder-to-shoulder with our oldest ally” in the American-led fight against global terrorism even if the Iraq War is “bitterly controversial.”

“For many it simply isn’t and can’t be worth it. For me, I think we must see it through,” Mr. Blair said. “They, the terrorists who threaten us here and around the world, will never give up if we give up. It is a test of will and of belief and we can’t fail it.” He added: “Hand on heart, I did what I thought was right. … I may have been wrong. That’s your call. But believe one thing, if nothing else: I did what I thought was right for our country.”

He concluded his subdued speech by offering “apologies for the times I have fallen short” and wishing his supporters “good luck.”

Mr. Blair’s decision also sets the stage for a six-week farewell tour to at least six nations, reportedly including a visit to Washington, that will mark the end of a historic era which brought Britain new optimism and opportunity but also anger, fear and war. Mr. Blair, who took office in May 1997 promising “a new dawn,” outlasted every other European leader in power at the time, and established himself as one of the world’s most visible and senior statesman, even though he will leave office at just 54.

“He’s going out on his own terms, with his head held high and a tremendous record of achievement behind him,” said Northern Ireland Secretary Peter Hain, just after a 15-minute cabinet meeting yesterday morning at 10 Downing Street where Mr. Blair advised his cabinet of his intentions. Mr. Hain said the meeting’s mood was light, and that “Tony cracked a few jokes.”

A memo written by Mr. Blair aides, which was disclosed in September, set out a strategy for Mr. Blair’s final days, saying, “He needs to go with the crowds wanting more. He should be the star who won’t even play that last encore.” News reports here said that in the coming weeks, Mr. Blair, who is famous for his showman’s panache, plans a final global lap that will take him away from disillusioned Britain and out into Europe, Africa and America, where his star is far less dimmed.

Mr. Blair reportedly will travel Friday to France to meet with president-elect Nicolas Sarkozy, who embodies a shift toward more conservative leadership that has swept across Europe, in countries such as Germany and Poland, in Mr. Blair’s later years in office. News reports also say that Mr. Blair intends later this month to visit Africa, a continent where he has worked insistently to fight poverty and disease. The trip will include a stop in South Africa, the land of a personal hero, Nelson Mandela, whose photograph Mr. Blair keeps on his desk alongside family portraits.

Grayer and thinner than on his early trips to see President Clinton, Mr. Blair also plans to travel to the White House to make a final call on President Bush, a man who is inextricably tied to Mr. Blair’s legacy. His relationship with Mr. Bush, which many in Britain see as a sort of subservience that has not benefited their country, and his steadfast defense of the Iraq war, has come to largely overshadow his other achievements.

Despite leading Labor to three national election victories, presiding over a decade of unbroken economic growth and exerting strong global leadership on issues such as climate change and poverty, Mr. Blair has not been able to escape widespread and caustic criticism in Britain that he is “Bush’s poodle.”

In his speech yesterday, Mr. Blair touted his domestic record and said no other British government since World War II had been as successful on the economy, education and health. But he said the most difficult decisions he made involved Iraq.

“Putting the country first does not mean doing the right thing according to conventional wisdom or the prevailing consensus, or the latest snapshot of opinion; it means doing what you genuinely believe to be right,” Mr. Blair said, adding, “Sometimes, you are alone with your instincts.”

Mr. Blair said removing the Taliban from Afghanistan and Saddam Hussein from Iraq had been relatively easy.

“But the blowback since, with global terrorism and those elements that support it, has been fierce and unrelenting and costly,” he said.

Mr. Blair was under no legal obligation to leave office; he won reelection in 2005 and could have served until the next national election, which must be held no later than 2010. But even before the last election, in September 2004, Mr. Blair made a surprise announcement that he would not run for a fourth term. The speculation that followed came to dominate political discourse in Britain. Finally, in September 2006, battered by fading approval ratings and the resignation of eight junior members of his government who said the prime minister had lost the party’s confidence, Mr. Blair announced that he would leave office by September 2007 “in the interests of the country.”

“I would have preferred to do this in my own way,” said an uncharacteristically glum Mr. Blair. Mr. Blair has been silent on his future plans. He and John Major, who was also 54 when Mr. Blair defeated him in the 1997 elections, are the youngest former prime ministers in at least a century. There has been much speculation that Mr. Blair, whose wife, Cherie, is a successful lawyer, will follow the lead of Mr. Clinton, who was also 54 when he left office, and start a private foundation to work on issues he considers vital, perhaps including Middle East peace.