"Our understanding is that we will be receiving the letter next week," IAEA spokeswoman Melissa Fleming told AFP.
Iran agreed during an unprecedented joint visit by the British, French and German foreign ministers to Tehran on October 21 to sign up to the tougher inspections regime set out in an additional protocol to the international nuclear Non-Proliferation Treaty (NPT).
Iran's archfoe, the United States, has since expressed skepticism about the Islamic regime's readiness to deliver on its undertakings.
But Iran's ambassador to the IAEA, Ali Akbar Salehi, said Tuesday the letter to the IAEA had already been drafted and would be delivered in a "matter of days".
In September the IAEA had imposed an October 31 deadline on Iran to fully disclose details of its nuclear program and urged it to sign the additional NPT protocol.
If the IAEA judges that Iran is not complying with its international nuclear safeguard commitments, it could submit the issue to the UN Security Council, which could then impose punishing sanctions on the Islamic Republic.
Russia is helping Iran build a nuclear power reactor but some Western nations, particularly the US, fear Tehran will use some of the atomic material destined for the power station to develop nuclear weapons.
On Friday an Iranian official in Tehran rejected suggestions that his government might renege on its commitments to the international community on its nuclear activities.
"When we make an undertaking at the international level, we respect it," the official said, asking not be identified.
He said Iran was already allowing access to IAEA inspectors that went "beyond that required by the additional protocol".
"Once we have actually signed it, we will work within the framework laid down by the protocol," he said.
IAEA chief Mohamed ElBaradei said in an interview with German newspaper Frankfurter Allgemeine Zeitung published Friday that IAEA inspectors were currently able to go where they wanted in Iran.