Blair, who will be stepping down as Britain's premier after a decade in office, announced to Labour party workers and supporters in his Sedgefield constituency, in northeastern England, that he will file his resignation June 27.
With Britain's next national elections still three years away, Blair is quitting early over growing criticism from political opponents and the public for his support of the U.S. military campaigns in Iraq and Afghanistan, as well as for corruption scandals within his party.
In his Sedgefield speech, Blair reiterated he had acted in good faith when he joined the U.S. effort to oust dictatorial Iraqi leader Saddam Hussein and the Taliban regime.
"I decided we should stand shoulder to shoulder with our oldest ally, and I did so out of belief," he said. "Hand on heart, I did what I thought was right. I did what I thought was right for our country."
Blair won three straight elections for Labour - for the first time in the party's history - and moved it from the left end of the political spectrum toward the center.
But his and his party's image was marred by a serious funding scandal last year when it emerged Labour had received multi-million pound loans ahead of the 2005 general vote in exchange for peerage pledges. Blair was questioned as a witness in that cash-for-honors case, becoming the first serving prime minister to have been quizzed by police conducting a criminal probe.
Blair's final weeks in office will culminate in a series of farewell trips overseas, including to France, where he is to meet president elect Nicolas Sarkozy Friday, The Times newspaper said.
Next week, the outgoing PM is due in Washington D.C. for a meeting with President George W. Bush, his closest ally in the international political arena.
Later this month, Blair will make a tour of the Black Continent, with South Africa as its highlight.
These travel destinations reflect the main priorities of Blair's international policies. He has been a staunch supporter of the U.S. in its war on international terrorism and its invasion of Iraq.
Blair's pro-Washington policies caused a major rift with European neighbors, including France, but he has spent the last few years trying to rebuild bridges across the English Channel. He will be the first foreign leader to meet with Sarkozy following the French Conservative leader's win in the May 6 presidential runoff.
Africa was the focus of Britain's G8 agenda when it was holding the rotating presidency of the group of leading industrialized nations in 2005. Blair then set up an Africa task group to work out a plan of action for the world's richer nations to alleviate the poverty-stricken continent's debt burden and to stimulate its economic and social development.
Before resigning as prime minister in June, Blair will also attend a G8 summit in the German city of Heiligendamm and a European Union summit in Brussels.
Humanitarian and environmental causes are expected to be the focus of Blair's activities after his departure from 10 Downing Street.
According to The Daily Telegraph, he could become an international commissioner for climate change, poverty in Africa or settlement in the Middle East. The Times says he is also considering establishing an organization to promote global interfaith dialogue.
Treasury Secretary Gordon Brown is tipped as the most likely candidate to replace Blair as the Labor leader and, consequently, as the head of government.
Deputy Prime Minister John Prescott also announced his resignation as Labour's deputy leader Thursday. He was elected together with Blair in 1994, but his role in the party leadership has been largely that of a figurehead since 2006, when he found himself at the center of several high-profile scandals. Specifically, he was accused of accepting favors from U.S. businessman Philip Anschutz, who sought a contract to convert London's riverfront Millennium Dome skyscraper into a huge casino.
