United Nations nuclear monitors say Iran has admitted to misleading them over its experiments with plutonium.
The UN's nuclear watchdog is expected to confirm later that Iran continued experimenting with plutonium - a key component of atomic bombs - until 1998.
Iran had previously told the body it had ended its experiments in 1993.
Correspondents say these latest inconsistencies in Iran's account will fuel suspicions about the real aims of its nuclear programme.
Iranian presidential favourite Akbar Hashemi Rafsanjani has told the BBC that Iran did not report all nuclear work.
"It's possible that at times, Iran has not reported its activities," Mr Rafsanjani told the BBC's Newsnight television programme.
And he accused the UN watchdog, the International Atomic Energy Agency (IAEA), of neglecting its duty to help Iran make peaceful use of nuclear technology.
Mr Rafsanjani, a former Iranian president seen as a frontrunner in Friday's presidential election, insisted his country would not abandon its nuclear programme.
But, he said, there was no risk of war with the US because Iran was not pursuing a nuclear bomb.
According to a draft speech to be delivered on Thursday to the IAEA's board of governors, the agency's deputy director Pierre Goldschmidt will confirm that Tehran has changed its version of events.
Tehran has now admitted that experiments took place in 1995 and 1998 after the IAEA confronted it with its analysis of its plutonium samples, according to the draft speech obtained by Reuters news agency.
"In a letter dated 26 May, 2005, Iran confirmed the agency's understanding with regard to that chronology," the draft speech says.
It also says Iran had acquired sensitive technology that could be used to make nuclear weapons earlier than it originally stated.
Iran maintains its nuclear programme is purely for civilian use
"This is nuclear material and, yet again, when Iran's backed into a corner the story changes," a Western diplomat on the IAEA board of governors told Reuters.
Iran is seeking the closure of a two-year UN investigation into its nuclear programme, which it says is solely for peaceful civilian purposes.
The US has threatened to refer Iran to the UN Security Council for sanctions over what Washington says are plans to build a nuclear bomb.
In his interview with the BBC, Mr Rafsanjani said the US had recently indicated it was willing to work with Iran.
He said the US had lifted obstacles to Tehran's entry into the World Trade Organization, had given consent to carry out limited enrichment of uranium and had agreed to sell it plane parts.
Mr Rafsanjani also urged the US to "leave Iraq as soon as possible" and hand over the running of the country to Iraqis.
Iran was pleased with recent political developments in Iraq, he said, pointing to the formation of new government in Baghdad.
"We hope the situation continues to progress like this," he said. "If it does... it will be easier for the Americans to leave Iraq."
Mr Rafsanjani said Iranians who were dissatisfied with the political process in the country should be free to speak their minds.
"If they have reasonable points, we should accept them," he told the BBC. "If not, we should persuade them of our case."