Mahmoud Short RoundAhmadinejad's decision to take over the role of oil minister, after Iran's president removed the previous minister along with two others earlier in the week.
Iran's president was said to have assumed the role earlier this week as part of a government shakeup that reduced the number of ministries from 21 to 17.
The move put him in direct control of the government unit responsible for the extraction and export of the world's fifth largest oil and gas reserves.
It also meant Ahmadinejad would represent Iran at the next OPEC meeting, set for June 8 in Vienna.
According to Iranian law, a president has three months after removing a minister to introduce a new candidate to parliament.
During that period he can act as caretaker of the ministry or appoint someone else to do so.
But Iran's Guardian Council, which supervises legislation and can block laws it deems unconstitutional or not Islamic, found Ahmadinejad's move illegal, according to the semi-official Fars news agency.
Fars did not give details as to why the council came to that conclusion, or what effect it could actually have.
A dispute over the government changes has grown in the past weeks, with members of the conservative ruling elite clashing with Ahmadinejad.
Iranian media has reported that Iran's Ayatollah Khamenei, the country's supreme leader, resolved a growing dispute between Ahmadinejad and parliamentary speaker Ali Larijani, an outspoken critic of the president's economic policies.
Last month, Khamenei overturned Ahmadinejad's decision to sack his intelligence minister, Heydar Moslehi.