Colin Powell's accusation this week that Iran is trying to develop a nuclear ballistic missile was based on information that is from a single, unvetted source, the Washington Post claimed today.

A US official with access to the highly classified material is said to have told the newspaper that the information came from a "walk-in" source who approached US intelligence earlier this month.

The source brought more than 1,000 pages purporting to be Iranian drawings, including a nuclear warhead design and modifications to enable conventional Iranian ballistic missiles to deliver an atomic strike.

The accusation has prompted concern that the US may be tempted to embark on another foreign policy adventure on shaky evidence.

The newspaper said senior Bush Cabinet members, including Mr Powell, were briefed on the information last week. The material was stamped "No Foreign", meaning it was not to be shared with allies, although President Bush was said to have shared some with Tony Blair last week.

The Washington Post report will evoke memories in Britain of the two dossiers - one officially recognised as "dodgy", based on out-of-date information downloaded from the internet, and the other later shown by the Butler Inquiry to lack sufficiently rigorous sourcing - in which the Prime Minister made the case for joining the US-led invasion.

Mr Powell used similar intelligence in a February 2003 speech to the UN Security Council, in which he tried to convince the world that Saddam Hussein had to be removed from power by force. Much of what he said later turned out to be based on unreliable information.

The Washington Post said in its report that the US was trying to avoid the mistakes of the past. It reported today: "US intelligence officials have been combing the information carefully and with a wary eye, mindful of the mistakes made in trusting intelligence information alleging that Iraq possessed weapons of mass destruction.

"If the information on Iran were confirmed, it would mean that the Islamic republic is further along than previously known in developing a nuclear warhead and the means to deliver it."

Mr Powell's accusation on Wednesday, during a trip to South America, that Iraq was trying to build "delivery systems" for a future nuclear weapon, was the latest salvo in the war of words between Washington and Tehran over the nuclear issue.

The Bush Administration has repeatedly accused Iran of using its nuclear energy programme to screen its real aim of producing an atomic bomb. The Iranians deny it and the issue is expected to be hotly debated next week at a meeting of the International Atomic Energy Agency (IAEA).

"You do not have a weapon until you put it in something that can deliver a weapon," Mr Powell said.

"I am talking about information that says they not only have these missiles, but I am aware of information that suggests that they were working hard to put the two together."

In a separate report about Mr Powell's remarks, the Los Angeles Times quoted one Bush administration source as saying that officials "were surprised he went public on something that was weak and, because it was weak, was not supposed to be used".

Experts believe that Mr Powell was referring to the Shahab-3, a medium-range ballistic missile with a range of more than 800 miles that could hit major foreign cities such as Tel Aviv in Israel, Nicosia in Cyprus, and Ankara in Turkey.

Andrew Brookes, an expert on missile technology at the International Institute for Strategic Studies (IISS), said that Iran was locked in an arms race with Israel, the only nuclear power in the region which can launch atomic weapons from land, sea and air.

The Shahab-3, which means meteor in Farsi, was successfully test-fired two years ago and put into service with the elite Revolutionary Guards. Ali Shamkhani, the Iranian Defence Minister, said this week that the country was now ready to mass-produce the weapon.

The latest estimates by the IISS, however, suggest that the Iranians have only about six of the missiles, which are modern versions of the Soviet-era Scud.

Mr Brookes said that adapting the projectile to carry a compact, workable and reliable nuclear warhead would require big technological advances by the Iranians. "That is what the Iranians are working on, but I suspect they are a considerable way from having a workable warhead," he told The Times.

Speculation over the missile coincided with fresh allegations by an Iranian opposition group that Iran has already acquired blueprints for a nuclear bomb and fissile material.

The National Council of Resistance of Iran said on Wednesday that the Iranians bought the weapons-grade uranium and the bomb design from Abdul Qadeer Khan, the father of Pakistan’s atomic bomb, who was accused of selling the technology to clients around the world. "He gave them the same weapons design he gave the Libyans, (and) more in terms of weapons design," Farid Soleimani, a senior member of the group, said.

Mr Powell said that he had "seen intelligence that would corroborate what this dissident group is saying", but the allegations were rejected by Iran and Pakistan.

Iran has reached an agreement with Britain, France and Germany to suspend its nuclear-enrichment programme in exchange for political and economic incentives. The deal is supposed to come into effect next week.

Explore World News

• World Agenda

• Iraq News

• US & Americas News

• Europe News

• Middle East News

• Asia News

• Africa News

• Ireland News

Times Recommends

• The Japanese Prime Minister is doomed: or is he?
• Brighton company 'involved in cyber attack'
• The art of survival: it's all in the mind