Iran's chief negotiator with the West over Tehran's nuclear programme, Ali Larijani, has resigned.
Mr Larijani has led negotiations with the West
A government spokesman said Mr Larijani had repeatedly offered his resignation and President Mahmoud Ahmadinejad had finally accepted it.
Mr Larijani had differences with the president over how to proceed with the negotiations, correspondents say.
Western countries suspect Iran of trying to build nuclear weapons but Tehran says its programme is peaceful.
The spokesman, Gholam Hossein 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.
Born in 1965
Is close to President Ahmadinejad
Considered a hard-line diplomat
Appointed deputy foreign minister in charge of Europe and American affairs in 2005
Has worked in Supreme Leader's office
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.
Although Mr Larijani is a conservative who was appointed by Mr Ahmadinejad to be Tehran's point man on the nuclear issue, his successor is known to be a close ally of the president.
The UN Security Council is waiting for reports from the EU's Mr Solana and from the UN's nuclear watchdog, the IAEA, before deciding on a third round of tougher sanctions against Iran.
Iran is developing the technology to enrich uranium on an industrial scale. The enriched uranium can be used as fuel in a nuclear power station.
Some Western countries, led by the US, fear Iran will further process the enriched uranium for use in nuclear weapons.
The IAEA says there are outstanding questions about Iran's nuclear activities but that it has recently reached agreement with Tehran on a "work plan" to resolve those issues.