American intelligence agencies based this week's surprise assessment that Iran had halted its nuclear weapons programme in 2003 on notes obtained from Iranian military officials.

These included vehement complaints about a decision to shut down what had been an active programme to build a nuclear warhead.

The notes were corroborated by other intelligence, US officials say, including intercepted conversations among Iranian officials, collated in recent months.

The CIA was so concerned that the notes might be part of an elaborate disinformation effort by Tehran that it organised a “red team” to check their veracity.

Once US intelligence officials decided that they were genuine, the notes and other evidence were presented two weeks ago in a White House meeting to the most senior members of President Bush's national security team, including Dick Cheney, the Vice-President, according to the New York Times.

In the meeting — described by one official as a “vivid exchange” — the intelligence officials were challenged to defend their new assessment that Iran's nuclear programme had been suspended.

Mr Bush and Mr Cheney have both sounded ominous warnings in recent months about Tehran's pursuit of nuclear weapons. As recently as October Mr Bush warned that a nuclear-armed Iran could lead to a third world war.

The spy operation caused a dramatic reversal of a 2005 US intelligence assessment that stated with “high confidence” that Iran was pursuing a nuclear weapon. The latest estimate, released on Monday, states with “high confidence” that Iran halted the programme in 2003 and has not restarted it. It also says, however, that Tehran continues its efforts to enrich uranium.

One of the great questions left unanswered by the US intelligence agencies is why Iran decided to suspend the programme in 2003. Monday's National Intelligecne Assessment said that it was “primarily in response to increasing international scrutiny and pressure”, but no firm conclusions about Tehran's motives appear to have been reached.

The revelation has reduced Mr Bush's chances of winning a tough, third set of UN sanctions against Tehran, which the US continues to pursue. It also appears to remove the casus belli for a US military strike.

China, a permanent member of the Security Council, indicated today that it would oppose a new round of economic sanctions. European Union diplomats said that fresh sanctions were now unlikely.

The intelligence finding has unnerved Arab nations in the Middle East because they believe that the receding prospect of a US military strike will embolden Iran. Although countries such as Saudi Arabia, Jordan and Egypt oppose a military strike, they welcomed the threat of one as it served to preoccupy Tehran and curb its ambitions in the region.

The Sunni Arab nations view Shia Iran as the greatest strategic threat in the Middle East. With further UN sanctions also unlikely, they do not disagree with President Ahmadinejad's declaration that the US intelligence assessment was a “victory” for Iran.

Questions still remain as to when Mr Bush learnt of the new intelligence, in light of the warnings that he has made repeatedly about Iran throughout the autumn.

The President said on Tuesday that he was briefed on the new findings only on Wednesday of last week. He also said, however, that in August Mike McConnell, the National Intelligence director, had informed him that there was new intelligence on Iran, but “he didn't tell me what the information was”.

The White House acknowledged today that Mr McConnell told Mr Bush in August that the new information might show that Iran's weapons programme “may be suspended” — but that the data needed to be examined and analysed.

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