Iran has temporarily recalled its ambassador to Britain amid an escalating dispute between the two countries.
Iranian exiles in the UK also called for Soleimanpour's extradition
The foreign ministry in Tehran said Ambassador Morteza Sarmadi had returned for "consultations", but did not specify how long he would stay away.
He is said to have been recalled after failing to win concessions following the arrest of another Iranian diplomat in Britain, Hade Soleimanpour.
Mr Soleimanpour's extradition is being sought by Argentina in connection with the bombing of a Jewish centre in Buenos Aires in 1994, when he was Iranian ambassador there.
The Foreign Office in London denied that Mr Sarmadi's departure amounted to a downgrading of relations.
Iran has threatened to withdraw some of its diplomats, but not its ambassador to London, over the arrest.
The BBC's Jim Muir in Tehran says it is unclear what actions the Iranian authorities are planning to take.
There is speculation that they are considering expelling the British ambassador to Iran, our correspondent adds.
Relations between the two countries have been strained since Mr Soleimanpour's arrest on 21 August, following an extradition request from Argentina.
The Argentine authorities believe he was involved in planning and commissioning the Jewish centre bombing, which killed 85 people.
He has strenuously denied any involvement, but has been refused bail after his arrest in Durham, where he was a research student at the city's university.
Iranian President Mohammed Khatami has demanded Mr Soleimanpour's release and an apology from Britain.
Last month, Iranian Deputy Foreign Minister, Ali Ahani, visited London to discuss the matter with UK Foreign Secretary Jack Straw.
The British Government says it cannot intervene in what it calls a purely judicial, and not political, process.
Mr Soleimanpour was on sabbatical from the Iranian embassy when he was arrested.