The International Atomic Energy Agency (IAEA) reported a series of breaches by Iran of international nuclear monitoring agreements, according to a text of the confidential document read to AFP by diplomats.
"It doesn't pull any punches. In exquisite detail, it shows step by step how Iran has lied to the agency," a Western diplomat said.
The United States accuses Iran, which is building a nuclear power reactor with Russian help, of secretly trying to develop nuclear weapons. Tehran vehemently denies the charge.
The report said Iran had "concealed many aspects of its nuclear activities with resulting breaches of its obligation to comply with the provision of the safeguards agreement" of the nuclear Non-Proliferation Treaty (NPT).
But it said: "There is no evidence that the previously undeclared nuclear material and activities ... were related to a nuclear weapons program."
The IAEA is still investigating the possibility that Iran is hiding an atomic weapons program, said the report, which is to be submitted to a meeting next week of the agency's 35-nation board of governors, which could declare Iran in non-compliance with the NPT.
This could lead to UN sanctions against Iran. But some diplomats said the country may escape a non-compliance ruling as it has over the past month yielded to key IAEA demands.
Others however pointed out the particularly harsh nature of the report which documents failures to report uranium imports and enrichment of uranium that could possibly be weapons-grade.
The IAEA discovered "the use of imported natural uranium hexafluoride for the testing of centrifuges at the Kalaye Electric Company (near Tehran) in 1999 and 2002, and the consequent production of enriched and depleted uranium," the report said.
The report said Iran had also failed to report "the import of natural uranium in 1994."
Iran has since October "adopted a policy of full disclosure and decided to provide the agency with a full picture of all its nuclear activities," the report said.
"Since that time Iran has shown active cooperation and openness. This is a welcome development," it said.
It said that until October, Iran's cooperation had been "limited and reactive."
The IAEA in September had asked Iran to do three main things ahead of the November 20 meeting: fully disclose its nuclear programme, agree to tougher inspections and suspend the enrichment of uranium that could be used to make an atomic bomb.
The main dispute is over traces of highly enriched uranium IAEA inspectors found at two sites in Iran.
The United States claims that Iran was using centrifuges to make highly enriched uranium but the Iranians claim the particles came from contamination from equipment they bought abroad.
Iran submitted on October 23 what it said was a full report on its nuclear program, only eight days before an October 31 deadline set by the IAEA for full disclosure.
And Iranian ambassador to the IAEA Ali Akbar Salehi handed over Monday to IAEA chief Mohamed ElBaradei a letter pledging to sign an additional protocol to the NPT for wider inspections and told him that Iran was as of Tuesday suspending its enrichment program.
Iran had told foreign minsters from Britain, France and Germany it would cooperate when the three diplomats visited Tehran on October 21 to break the deadlock.
A Western diplomat said that "after the visit to Tehran by the three foreign ministers and this report, it is most likely there will not be a finding of non-compliance at the board of governors but this will be for political reasons."
He said Iran's failures to honor safeguards were still "enough evidence for the board to adopt a resolution of non-compliance."