The head of Iran's Atomic Energy Organisation, Gholam Reza Aghazadeh, has resigned. He was appointed in 1997.
The ISNA news agency says he submitted a letter of resignation to President Mahmoud Ahmadinejad nearly three weeks ago. It was not immediately clear what triggered the resignation.
Iran says its nuclear programme is for civilian power, denying Western claims that it wants a nuclear bomb.
ISNA reported Mr Aghazadeh also stepped down as Iran's vice-president.
The agency said Mr Aghazadeh has given no explanation for his resignation, which has been accepted by Mr Ahmadinejad.
Mr Aghazadeh is a veteran official who served in the 1980s as a deputy to Mir Hossein Mousavi - the defeated candidate in Iran's disputed presidential elections last month.
He was appointed oil minister in 1985, staying in the post for 12 years during the presidency of Ali Akbar Hashemi Rafsanjani.
He then moved to his job at the head of the atomic agency in 1997 under former president, Mohammad Khatami.
He continued in the post when Mr Ahmadinejad was first elected in 2005.
The BBC reports it is not clear whether Mr Aghazadeh's resignation was linked to the disputed presidential elections on 12 June.
Control over Iran's foreign and nuclear policies ultimately lies with Supreme Leader Ayatollah Ali Khamenei.