Sept. 19 (Bloomberg) -- Republican presidential candidate Rudy Giuliani threatened to put Iran ``back five years'' if the Islamic Republic gets close to developing nuclear weapons.

``I believe the U.S. and its allies should deliver a very clear message to Iran,'' Giuliani told reporters today during a visit to London to deliver a lecture and meet with U.K. leaders. ``They are not going to be allowed to become a nuclear power. It is just not going to happen.''

Iran's claim that it wants nuclear technology to generate electricity has failed to convince the U.S., U.K. and other western governments, which suspect Iran's government is trying to develop nuclear weapons. The United Nations has imposed two sets of limited sanctions since December, and the U.S. is leading calls to toughen them unless Iran halts uranium enrichment.

President George W. Bush has refused to rule out taking military action to prevent Iran's building an atomic bomb, and French Foreign Minister Bernard Kouchner said last week that the world should ``prepare for the worst'' if Iran gains such a weapon.

Giuliani said it is ``too risky'' to allow let Iran build nuclear devices.

``If they get to the point that they become a nuclear power, then we will set them back five years. That is not a threat, that is a promise,'' Giuliani, 63, said.

`Fantasy' Role

Iran ``has displayed too often irresponsible behavior and has even displayed a kind of fantasy about their role in the world,'' the former New York City mayor said.

Iran said today it has drawn up plans to bomb Israel if it comes under attack from the Jewish state.

Giuliani, who was New York mayor when the city was attacked on Sept. 11 and has made a tough approach to dealing with terrorism a centerpiece of his presidential campaign, spoke after talks with Prime Minister Gordon Brown today.

``Mayor Giuliani asked to pay a call on the prime minister, and given his experience in New York and counter-terrorism, the prime minister thought it would be a good idea to see him,'' Michael Ellam, Brown's spokesman, told reporters.

Later, Giuliani delivered the first Margaret Thatcher- Atlantic Bridge lecture. The Atlantic Bridge aims to strengthen policy and personal ties between ``like-minded conservatives'' in the U.S. and U.K., according to the institute's Web Site.

Expanding NATO

Giuliani told the audience, which included Thatcher and other U.K. Conservative lawmakers, that the North Atlantic Treaty Organization should be expanded to include new countries.

``NATO has been a vital international institution,'' Giuliani said. ``We should open the organization to any country who meets basic standards of good governance.''

Those countries could include Australia, Singapore, Israel, India and Japan, he added. He also criticized current NATO countries that didn't ``commit'' their troops to military operations in Afghanistan.

``It's maybe time for the older members of NATO to reconsider their commitment to NATO,'' he said. ``Maybe an expansion of NATO would be a very, very good idea in getting the job done in Afghanistan.''

With 7,700 troops in Afghanistan, Britain has the second- biggest contingent of foreign soldiers in the country behind the U.S. NATO has a total of 36,000 troops in Afghanistan battling the resurgent Taliban, the radical Islamic movement toppled from power by the U.S. in 2001.

To contact the reporter on this story: Kitty Donaldson in London at kdonaldson1@bloomberg.net.

Last Updated: September 19, 2007 18:09 EDT