Some Iranians want the UK ambassador expelled from Tehran
The Foreign Office is to allow non-essential staff and their families to leave the British embassy in Tehran, Iran, following a gun attack on the building.
But the department made it clear in Thursday's announcement that there is no threat to Britons travelling to Iran or to the British community in the country. They are not being advised to leave.
A Foreign Office spokeswoman said authorisation to leave had been given because of an "increased threat" to the embassy following the attack.
She added: "We are not advising the British community to leave, nor are we advising against non-essential travel to Iran as we believe the threat is against the embassy and not private individuals.
"This is a straightforward security measure. We are not downgrading diplomatic relations."
The announcement follows the closure of the embassy after shots were fired at the building from a nearby street on Wednesday. Five shots hit the embassy, but nobody was hurt in the attack.
Foreign Ministry spokesman Hamid Reza said police had put in place special surveillance and security measures around the embassy and were following up very seriously "these irresponsible actions".
The shooting followed an announcement that Iran had temporarily recalled its ambassador to Britain amid an escalating dispute between the two countries.
Iran's ambassador to Britain, Morteza Sarmadi, was recalled after allegedly failing to win concessions following the arrest of another Iranian diplomat in Britain, Hade Soleimanpour.
21 August: Hade Soleimanpour arrested in northern England, following a request from Argentina
24 August: Iran protests at the arrest and demands an apology
27 August: Iranian deputy foreign minister travels to London, but UK says Mr Soleimanpour's fate is a matter for the courts
3 September: Iran recalls its ambassador 'for consultations'
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 blast killed 85 people.
Relations between Britain and Iran have been strained since Mr Soleimanpour's arrest on 21 August.
Iranian President Mohammad Khatami has demanded Mr Soleimanpour's release and an apology from Britain.
But the British Government says it cannot intervene in what it calls a purely judicial, and not political, process.
Britain and Iran resumed full diplomatic relations at ambassadorial level in 1999 after a long break following the overthrow of the shah in the 1979 Islamic revolution.