The Islamic republic has faced accusations it is using its civil nuclear energy programme as a cover to develop nuclear weapons -- a charge it vehemently denies -- and last month the UN agency condemned Tehran for 18 years of covert nuclear activities.
But an International Atomic Energy Agency (IAEA) report said there was no clear evidence the country had been developing nuclear arms.
"It is confirmed. They will sign," IAEA spokesman Peter Rickwood told AFP on Wednesday.
"It is an historic event," he said.
Rickwood said the signing of an additional protocol to the Nuclear Non-Proliferation Treaty (NPT) would take place at the agency's headquarters in Vienna at 3:00 pm (1400 GMT) in the presence of IAEA Director General Mohamed ElBaradei.
Iran's vice president and head of its atomic energy organization, Gholamreza Aghazadeh, earlier from Tehran signalled that the Islamic republic could sign the protocol Thursday, telling reporters: "Possibly tomorrow."
Iranian diplomats in Vienna said late Wednesday it was not yet clear who would sign the protocol on behalf of Tehran.
The United States has put some of the strongest pressure on Tehran to sign the text, accusing it of trying to build a nuclear bomb.
ElBaradei, meanwhile, has argued that the agency needs more powers as it continues to probe Iran, including the ability to carry out unannounced inspections of suspect nuclear sites.
Under the basic treaty, the IAEA is only authorised to carry out pre-arranged site inspections.
Iran, named as part of an "axis of evil" by US President George W. Bush along with North Korea and the ousted regime in Iraq, had long been resisting the additional protocol.
It argued that inspectors could violate national sovereignty and probe sites that are crucial to the defence of the country and demanded guarantees that this would not happen.
But Iran finally gave in to IAEA demands after the agency threatened to refer its concerns to the UN Security Council, which would have left Iran vulnerable to sanctions.
That U-turn came in October during an unprecedented visit by the foreign ministers of the European Union's big three -- Britain, France and Germany.
Iran agreed to sign the additional protocol, hand over full details of its activities and suspend uranium enrichment.
Russia kept the pressure up this week with Atomic Energy Minister Alexander Rumyantsev warning Monday that Russia would not deliver nuclear fuel to Iran for a new atomic power plant unless Tehran signs the protocol.
Moscow, under US pressure to stop its nuclear assistance to Iran, is preparing an agreement with Tehran under which Iran pledges to return all spent fuel from the nuclear reactor that Russia is building at Bushehr, in the south of the country.
Supreme leader Ayatollah Ali Khamenei, who has the final say on all matters of state, has given his blessing to the additional protocol.
But a number of influential conservatives have voiced strong opposition to allowing the tougher inspections.
Several had gone so far as to advocate following the path of fellow "axis" member North Korea and pulling out of the NPT, which took effect in 1970, altogether.
The Iranian government has vehemently denied that it is seeking nuclear arms, saying the only aim of the nuclear programme is to allow it to provide cheaper energy to its people.