"Iran must not be allowed to acquire nuclear weapons capability. Iran must know that we will do whatever it takes to stop them getting it," Blair told the pro-Israel lobby AIPAC.
"The danger is if they suspect for a moment we might allow such a thing," he told delegates on the last day of the three-day annual policy conference of AIPAC, the American Israeli Public Affairs Committee.
"We cannot and we will not. This is not simply an issue of Israel's security. This is a matter of global security, mine yours, all of us," the former British prime minister said.
"Iran's regime is the biggest destabilizing influence in the region," he said, adding that both Israelis and Arabs know this.
He was alluding to Iran's support for anti-Israeli Muslim militant groups in Lebanon and in the Palestinian territories as well as to Shiite militants in Iraq, which was formerly led by Sunnis, the majority group in the Arab world.
Yemen has also accused Shiite northern rebels of taking money from Iranians and of plotting to create a Shiite zone along the Saudi borders.
During the AIPAC policy conference, Israeli Prime Minister Benjamin Netanyahu and US Seccretary of State Hillary Clinton raised alarms about the perceived nuclear threat from Iran.
Clinton said it was worth taking the time needed for the United Nations Security Council to adopt new sanctions "that bite" against Iran.
Israel, which sees the threat more urgently, has raised the threat of a pre-emptive military strike against Iran's nuclear sites.
The United States, Israel and others fear that Iran's uranium enrichment program masks a drive for a nuclear bomb. Tehran denies the charge, saying it is for peaceful nuclear energy.
