LONDON - Prime Minister Tony Blair passionately defended his decision to join the U.S.-led war on Iraq, saying today that governments could not "err on the side of caution" when dealing with threats of global terrorism and weapons of mass destruction.

Blair, whose popularity at home has slumped since the campaign to topple Saddam Hussein, said the decision to commit Britain to the war was his most divisive since becoming leader in 1997.

But, in a speech to an invited audience in his constituency of Sedgefield, northern England, Blair argued that the international community had a "duty and a right to prevent the threat materializing" and to stop a regime brutally oppressing its people.

Almost a year after the U.S.-led invasion of Iraq, Blair continues to pay the price for supporting President Bush.

Despite the swift fall of Baghdad, Blair's personal ratings have failed to recover, the debate about whether the war was legal still rages and critics continue sniping about the coalition's failure to find weapons of mass destruction in Iraq.

Blair has tried to focus public attention on his domestic agenda of improving public services. But claims the government exaggerated the threat posed by Saddam to justify the war, the suicide of a weapons expert at the center of those allegations and claims of a British spying operation at the United Nations have jolted the government.

While Blair offered no new arguments for the war today, he offered a wide-ranging defense of his judgment.

He argued that the "nature of the global threat we face in Britain and round the world is real and existential and it is the task of leadership to expose it and fight it, whatever the political cost."

Blair said people were in "mortal danger of mistaking the nature of the new world in which we live."

"The threat we face is not conventional. It is a challenge of a different nature from anything the world has faced before. It is to the world's security what globalization is to the world's economy," Blair said.