The disclosure makes it harder for President George Bush and the vice-president, Dick Cheney, to make a case for a military strike against Iran next year.

It also makes it more difficult to persuade countries such as Russia and China to join the US, Britain and France in imposing a new round of sanctions on Tehran.

The national security estimate, which pulls together the work of the 16 US intelligence agencies, today published a declassified report revising previous assessments of Iran's weapons programme.

"Tehran's decision to halt its nuclear weapons programme suggests it is less determined to develop nuclear weapons than we have been judging since 2005," it said.

Bush and Cheney have been claiming that Tehran is bent on achieving a nuclear weapon. The British government, which is planning to discuss the report with its US counterparts over the next few days, has also repeatedly said it suspects Iran of seeking a nuclear weapons capability.

The Iranian government insists it is only pursuing a civilian nuclear programme.

The US national security estimate disclosed that Tehran had halted its nuclear weapons programme in 2003 and had not restarted it.

Two years ago, the national security estimate reached a different conclusion, saying Iran was still developing its nuclear weapons programme.

The White House continued to claim today that Iran remains a threat to the region and the world as a whole.

Although a halt to nuclear weapons development is significant, the national security estimate is far from a clean bill of health for Iran. The country is pushing ahead with its uranium enrichment programme, which has only limited civilian use and could be quickly converted to nuclear military use. The national intelligence estimate warned that Iran could secure a nuclear weapon by 2010.

The US state department's intelligence and research office says the more likely timescale would be 2013. All the agencies concede that Iran may not have enough enriched uranium until after 2015.

Referring to Iran's decision to halt the military programme in the autumn of 2003, Stephen Hadley, the White House national security adviser, said today: "Today's national intelligence estimate offers some positive news. It confirms that we were right to be worried about Iran seeking to develop nuclear weapons. It tells us that we have made progress in trying to ensure that this does not happen.

"But the intelligence also tells us that the risk of Iran acquiring a nuclear weapon remains a very serious problem. The estimate offers grounds for hope that the problem can be solved diplomatically, without the use of force, as the administration has been trying to do."

Hadley said the White House would continue to try to intensify international pressure on Iran. Russia and China, two of the permanent members of the UN security council, have scuppered attempts by the US over the last six months to impose tough new sanctions on Iran.

Bush and Cheney stepped up rhetoric against Iran this year. Bush said in October: "If you're interested in avoiding world war III, it seems like you ought to be interested in preventing them from having the knowledge necessary to make a nuclear weapon." Cheney warned of "serious consequences" if the government in Tehran did not abandon its nuclear programme.

The decision to publish the national intelligence estimate is aimed at trying to recover the public credibility lost when the agencies wrongly claimed Iraq had weapons of mass destruction in the years running up to 2003. A US official said this time "there was a very rigorous scrub using all the trade craft available, using the lessons of 2002".

Mitch McConnell, the director of national intelligence, last month decided that its reports would not longer be released automatically. But an exception is being made in this case because the new conclusion contradicts the 2005 findings.

In a separate statement accompanying the report, the deputy director, Donald Kerr, said that given the new conclusions, it was important to release the report publicly "to ensure that an accurate presentation is available".

The national intelligence report says that Iran would have to go flat out to secure enough highly enriched uranium by 2010. The state department's intelligence and research office says that the more likely timescale would be 2013. All the agencies concede that Iran may not have enough enriched uranium until after 2015.

The report says that ultimately Iran has the technical and industrial capability to build a bomb "if it decides to do so" but it Iran may be amenable to pressure. Tehran's "decisions are guided by a cost-benefit approach rather than a rush to a weapon irrespective of the political, economic and military costs."

It adds: "Some combination of threats of intensified international scrutiny and pressures, along with opportunities for Iran to achieve its security, prestige, and goals for regional influence in other ways might - if perceived by Iran's leaders as credible - prompt Tehran to extend the current halt to its nuclear weapons programme."