John Chapman, director of the London-based International Institute for Strategic Studies, told a news conference it was unlikely that diplomatic pressure from the European Union would stop Iran developing its nuclear enrichment programme.
He said if Iran ended up with a confirmed, deployed nuclear capability, Turkey, Egypt and Saudi Arabia would "reconsider their positions".
Western nations suspect Iran is developing atomic weapons under cover of a civilian programme although Tehran insists it is intended only for peaceful electricity generation.
The United States and European Union have put diplomatic pressure on Iran to halt its programme and are also trying to persuade the International Atomic Energy Agency (IAEA) to refer Iran to the United Nations for possible sanctions.
The IISS said in its annual handbook on global military might, "The Military Balance", released on Tuesday that it was unlikely the EU diplomacy would yield results.
However, Chapman said a nuclear Iran was still a long way off so there was time to focus on diplomatic activity.
Russia and China oppose the referral of Iran to the Security Council saying that uranium conversion is a step short of the actual enrichment needed to produce weapons.
"It would be desirable for regional states, especially the Gulf Arab states, also to express more openly their known concerns about how Iran's possible acquisition of a nuclear capacity would change strategic perceptions and the regional balance of power," said Chapman.
The United States said this week military action against Iran was not on the agenda but that President George W. Bush would not rule out any option while the international community pursued diplomatic means.