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By Stefano Ambrogi

LONDON, Dec 19 (Reuters) - Iran is unlikely to disrupt oil shipments through the Strait of Hormuz in the Gulf in any conflict with the West over its nuclear ambitions, but the risk remains, reinforced by Tehran's bellicose rhetoric.

Any military interference in the strategic channel at the entrance to the Gulf could double oil prices in days and deal a blow to the world economy, maritime and security experts say.

Attacks on merchant shipping in the international waterway, which shares Iran's coastline, would disrupt exports from OPEC's biggest producers and knock the booming economies of Gulf states.

The waterway, just 34 miles (55 km) across at its narrowest point, is a thoroughfare for 33 percent of the world's seaborne crude oil exports, according to Lloyd's Marine Intelligence Unit (LMIU), citing 2006 figures.

The oil flow, which fluctuates with OPEC production, is closer to 40 percent when it includes carriage of petroleum products and is calculated as a percentage of globally traded oil.

Defence, trade and maritime experts, who gathered at a Royal United Services Institue (RUSI) conference on the strait's security in London late last week, said any attempt to impede passage would reverberate around the world.

"The psychological impact of a perceived threat to the strait alone is great -- even the suggestion of a restriction stokes oil prices," RUSI Director Michael Clarke told the conference.

Closure to merchant shipping or attacks on oil tankers going through the passage would kick oil prices -- above $90 a barrel and near all-time records -- higher still.

Many oil analysts reckon prices could easily double until it reopened. Risk and credit agency Standard & Poor's says prices could rocket to $250 a barrel on any blockage.

But analysts say the probability of Iran attempting to block the strait is low -- even the U.S. military says there is a slim chance of it happening.

"Iran is by far the most dependent for exports of its hydrocarbons through the strait. So there is a striking illogic in this," said Richard Schofield, an expert in international boundaries at King's College in London.

Iran has explicitly threatened to close the strait before, during the Iran-Iraq war from 1980-1988, when the West and especially the United States began backing Iraq.

During the conflict Iran and Iraq attacked merchant shipping in the Gulf, including those of neutral nations. Dubbed the "Tanker War" it is a reminder of what could happen, experts say.

"More than 500 ships were attacked with over 400 civilians killed. Iranian oil exports were cut by 50 percent," said Mark Hankey of LMIU.

ECONOMIC IMPACT

The impact on Gulf economies of a military exchange in or close to the strait would be disastrous, while Japan, South Korea and the United States' oil supplies would be hit hardest.

Please see Factbox on economic consequences [ID:nL19139480]

According to LMIU, closure would knock out 88 percent of Saudi Arabia's seaborne crude exports, all of the supply from the United Arab Emirates, Kuwait and Qatar, almost all of Iraq's and 90 percent of Iran's crude shipments.

Japan, which sources 26 percent of its crude oil through the passage, would suffer most, according to LMIU -- the shipments meet 85 percent of its oil needs.

THE OIL WEAPON

Iran's President Mahmoud Ahmadinejad has publicly stated that Tehran would not use oil as a weapon even if it were attacked by the United States in a pre-emptive strike over its disputed nuclear programme.

But analysts say statements from Iran are often contradictory, setting off alarm bells in Washington and unnerving Gulf countries whose booming economies are heavily reliant on trade through the strait.

Mustafa Alani, director of security and terrorism at the Dubai-based Gulf Research Center, said Iran's Revolutionary Guards Corps and military officials have made more than 20 references to closing the strategic corridor if attacked.

Only in late October a senior Iranian naval commander said the "martyrdom-seeking" Basij militia could target oil shipments through the strait and wider Gulf waters.

U.S. Naval Central Command, headquartered in Bahrain, has pledged to keep the international waterway open at all costs, regularly moving aircraft carrier strike groups in and out of the Gulf and drilling on Iran's doorstep.

In December, Vice Admiral Kevin Cosgriff, head of U.S. Naval Forces Central Command, called Iran's gesturing on the strait irresponsible and intimidating.

"I wake up thinking about Iran, I go to bed thinking about Iran," he said. (Editing by Anthony Barker)

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