London ? Iran is two to three years away from having the capacity to build a nuclear weapon, a leading security think tank said Wednesday.

But the London-based International Institute for Strategic Studies said domestic opposition to outspoken President Mahmoud Ahmadinejad could still help put the brakes on its nuclear development efforts.

Both John Negroponte, the head of national intelligence for the U.S., and Mohamed ElBaradei, director general of the International Atomic Energy Agency, have said Iran is perhaps four years from the capacity to produce a nuclear weapon.

John Chipman, the institute’s chief executive, said Wednesday that Iran was on track to complete its goal of producing 3,000 centrifuges for producing highly-enriched uranium by the end of March or shortly thereafter.

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

Iran says it aims to produce nuclear fuel to generate electricity. But if Iran chose, it could use the massive array of centrifuges to make enough weapons-grade material for dozens of nuclear warheads a year.