Mr Bush has signed an official document endorsing CIA plans for a propaganda and disinformation campaign intended to destabilise, and eventually topple, the theocratic rule of the mullahs.

Under the plan, pressure will be brought to bear on the Iranian economy by manipulating the country's currency and international financial transactions.

Details have also emerged of a covert scheme to sabotage the Iranian nuclear programme, which United Nations nuclear watchdogs said last week could lead to a bomb within three years.

"Iranians in America have links with their families at home, and they are a good two-way source of information," he said.

The CIA will also be allowed to supply communications equipment which would enable opposition groups in Iran to work together and bypass internet censorship by the clerical regime.

The plans, which significantly increase American pressure on Iran, were leaked just days before a meeting in Iraq tomorrow between the US ambassador, Ryan Crocker, and his Iranian counterpart.

Tensions have been raised by Iran's seizure of what the US regards as a series of "hostages" in recent weeks. Three academics who hold dual Iranian-American citizenship are being held, accused of working to undermine the Iranian government or of spying.

An Iranian-American reporter with Radio Free Europe, who was visiting Iran, has had her passport seized. Another Iranian American, businessman Ali Shakeri, was believed to have been detained as he tried to leave Teheran last week.

The US responded with a show of force by the navy, sending nine warships, including two aircraft carriers, into the Persian Gulf.

Authorisation of the new CIA mission, which will not be allowed to use lethal force, appears to suggest that President Bush has, for the time being, ruled out military action against Iran.

Bruce Riedel, until six months ago the senior CIA official who dealt with Iran, said: "Vice-President [Dick] Cheney helped to lead the side favouring a military strike, but I think they have concluded that a military strike has more downsides than upsides."

However, the CIA is giving arms-length support, supplying money and weapons, to an Iranian militant group, Jundullah, which has conducted raids into Iran from bases in Pakistan.

Iranian officials say they captured 10 members of Jundullah last weekend, carrying $500,000 in cash along with "maps of sensitive areas" and "modern spy equipment".

Mark Fitzpatrick, a former senior State Department official now with the International Institute for Strategic Studies, said industrial sabotage was the favoured way to combat Iran's nuclear programme "without military action, without fingerprints on the operation."

He added: "One way to sabotage a programme is to make minor modifications in some of the components Iran obtains on the black market."

Components and blueprints obtained by Iranian intelligence agents in Europe, and shipped home using the diplomatic bag from the Iranian consulate in Frankfurt, have been blamed for an explosion that destroyed 50 nuclear centrifuges at the Natanz nuclear plant last year.

The White House National Security Council and CIA refused to comment on intelligence matters.