Iran is as little as two to three years away from building an atomic bomb, a leading security think tank reported Wednesday.

The estimate given by the International Institute for Strategic Studies is lower than that given by John Negroponte, the head of national intelligence for the United States, who said that Tehran could build such a weapon in as few as four years.

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The London-based institute said domestic opposition to Iran's outspoken president could still put a brake on its contentious nuclear program.

"There are signs that political and economic pressure is having an impact in Tehran," said John Chipman, the institute's chief executive, speaking at the launch of the its annual publication, "The Military Balance."

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Iran was on track to complete its goal of producing 3,000 centrifuges by the end of March or shortly thereafter, Chipman said, noting that a significant proportion of these had been obtained from the black market.

The International Atomic Energy Agency said Friday that Iranian officials had informally announced plans to install some 3,000 centrifuges at its underground nuclear facility in Natanz, in central Iran, sometime within the next month.

Iran ultimately plans to expand its program to 54,000 centrifuges, which spin uranium gas into enriched material.

In enrichment plants, centrifuges are linked in what are called cascades. Chipman said that cascades were much more difficult to complete than the centrifuges themselves.

"Getting the centrifuge cascades to function properly is then another task of an entirely different order of magnitude," he said, adding that the process could take at least a year.

Once the cascade was operational the institute predicted it would take another nine to 11 months to produce 55 pounds of highly enriched uranium, enough for a single bomb, he said.

Iran has said it needs the uranium to produce electricity — but the United States and some of its allies accuse Tehran of using the program to produce nuclear weapons.

Chipman also said it was possible that growing disquiet within Iran over President Ahmadinejad's leadership style — and the economic difficulties linked to possible sanctions against the country — might relaunch a domestic debate over the future of the country's nuclear program.

"Whether the internal debate will lead to a suspension in the enrichment program that would provide the basis for resumed negotiations remains to be seen," he said.

The institute is widely considered the most important security think tank outside the United States.