Iran's chief nuclear negotiator Ali Larijani has resigned, a government spokesman has said.
The spokesman, Gholam Hossein Elham, said Mr Larijani had repeatedly offered his resignation and President Mahmoud Ahmadinejad had finally accepted it.
Mr Larijani has been Iran's point man in negotiations with Western countries that suspect Iran of trying to build nuclear weapons.
Iran says its nuclear programme is for peaceful purposes only.
Mr Elham said a deputy foreign minister, Saeed Jalili, would replace Mr Larijani in time for a meeting on Tuesday with the European Union's foreign policy head Javier Solana.
The BBC's Jon Leyne, in Tehran, says Mr Larijani has had differences with President Ahmadinejad over how to proceed with negotiations over the country's nuclear programme.
Mr Larijani has favoured further negotiations with the West over Iran's uranium enrichment programme, as opposed to the president's more hard-line approach, our correspondent says.
The resignation comes just days after Russian President Vladimir Putin visited Tehran and expressed qualified support for Iran's right to a peaceful nuclear programme.
Some Iranian media reported that Mr Putin had offered new compromise proposals over the stand-off with some Western countries.
Media close to President Ahmadinejad, however, have denied that the Russian president made new proposals.
The resignation is a sign, says our correspondent, that Iran's supreme leader, Ayatollah Ali Khamenei, has thrown his weight behind President Ahmadinejad and his hard-line approach on the nuclear issue.