Hundreds of scientists and technicians would be targets in the opening salvos as the attacks focused on eliminating further nuclear development, the Oxford Research Group says in Iran: Consequences of a War.

The research coincides with reports that strategists at the Pentagon are drawing up plans for "a last resort" strike if diplomacy fails. Plans for an assault have taken on "greater urgency" in recent months, The Sunday Telegraph said.

Tacticians at central command and strategic command, who report to Donald Rumsfeld, the defence secretary, have been identifying targets and the weapons needed to hit them.

The Oxford report says that Britain could be drawn into the conflict if the Prime Minister allowed American B2 bombers, which can carry 40,000lb of precision bombs, to use bases at Fairford, Glos, and on the Indian Ocean island of Diego Garcia.

Precision bombing could put Iran's weapons programme back five to 10 years but within a month the situation would become "an extremely dangerous conflict", says Prof Paul Rogers, the report's author.

The attack would result in "a protracted military confrontation" involving Israel, Lebanon and some Gulf states.

More than 100 American bombers, many based on carriers in the Gulf, would take part in a huge simultaneous surprise air attack on 20 key nuclear and military facilities, the report says.

If the targets included the nuclear reactor at Bushehr, which will become fully fuelled this year, a radioactive cloud could spread over the Gulf. Iran's small navy, which includes three submarines, would have to be attacked to negate threats to vital shipping lanes in the Straits of Hormuz.

But Iran could still retaliate with suicide speedboats, possibly leading to crippling rises in the price of oil.

Prof Rogers, professor of peace studies at Bradford University, says that American military action would also have a unifying effect on the rule of President Mahmoud Ahmadinejad and exacerbate anti-American hostility in the Islamic world.

The report says that a ground offensive in Iran would not be feasible, as it would require at least 100,000 troops - and American forces are already over-stretched with 130,000 soldiers in Iraq and 18,000 in Afghanistan.

Iran would probably withdraw from the nuclear non-proliferation treaty and speed up its secret nuclear weapons programme.

The report concludes: "A military response to the current crisis is a particularly dangerous option and should not be considered further. Alternative approaches must be sought, however difficult these may be."

In a similar briefing before the invasion of Iraq in 2003, the Oxford group predicted that Saddam Hussein's regime could easily be overwhelmed but that the country would become a hotbed of insurgency.