Gholam Reza Aghazadeh was named by ex-President Mohammad Khatami
The long-serving head of Iran's Atomic Energy Organisation, Gholam Reza Aghazadeh, has resigned, Iranian media and officials have said.
The nuclear chief had submitted a letter of resignation to President Mahmoud Ahmadinejad nearly three weeks ago, the Isna news agency said.
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.
Mr Aghazadeh also stepped down as the country's vice-president, Isna reported.
The agency said it had spoken to Mr Aghazadeh, who gave no explanation for his move but told them that Mr Ahmadinejad had accepted the resignation.
A spokesman for Iran's atomic energy department and the official Irna news agency confirmed the news.
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.
In 1985 he began a 12-year stint as oil minister, staying in the post during the presidency of Ali Akbar Hashemi Rafsanjani.
He then moved to his job at the head of the atomic agency in 1997 under the reformist former president, Mohammad Khatami.
He continued in the post when Mr Ahmadinejad was first elected in 2005.
It was not clear whether Mr Aghazadeh's resignation was linked to Iran's 12 June disputed presidential elections, in which Mr Ahmadinejad claimed victory.
Mr Aghazadeh has made no public comment on the turmoil that followed the vote.
Correspondents say that although Mr Aghazadeh has long supervised Iran's nuclear programme, his resignation may have a limited impact on negotiations with Western powers, which are headed by Iranian nuclear envoy Saeed Jalili.
Control over Iran's foreign and nuclear policies ultimately lies with Supreme Leader Ayatollah Ali Khamenei.
The news of Mr Aghazadeh's resignation came a day after US Secretary of State Hillary Clinton warned Iran that the US would not extend its offer of engagement indefinitely.
President Barack Obama has talked of engagement with Iran but has not made clear how that might take place.