The protocol gives IAEA inspectors the right to make unannounced visits to suspect sites, even if the host country has not declared them as open to inspection.
"The board has approved Iran's additional protocol and now it is ready to be signed," said Melissa Fleming, spokeswoman for the International Atomic Energy Agency (IAEA).
IAEA chief Mohamed ElBaradei maintains that wider inspections are crucial to be able to guarantee that Iran is not secretly producing nuclear weapons.
The United States is accusing Iran of trying to develop nuclear weapons and wants the IAEA board to ask the UN Security Council to address the claims and possibly impose sanctions.
The additional protocol to the nuclear Non-Proliferation Treaty (NPT) was created in 1997 after the IAEA discovered that previous inspections had not been broad enough to uncover hidden nuclear activities in Iraq.
Iran promised on October 21 in an agreement with Britain, France and Germany to sign the additional protocol and delivered a letter formalizing this promise on November 10 to ElBaradei.
But Iran has refused at the IAEA board meeting to set a date for signing the protocol until its sees what sort of a resolution the board is to pass on Iranian violations over the past 18 years of NPT safeguards agreements.