By Jim Muir
BBC correspondent in Tehran
There has been another attack on the main British embassy compound in the Iranian capital, Tehran.
The British embassy in Tehran has been a target
An embassy spokesman said an incendiary device was thrown over the back gate of the compound but there were no casualties or significant damage.
The embassy was the target of at least three shooting attacks in the first half of September.
The attack was carried out during the afternoon on Friday, a day when the streets are quiet.
First reports said a lone man stepped out of a car, threw the firebomb and then drove off, something that would not have been possible on a normal working day when traffic is heavy.
An embassy spokesman said the device caused a small fire but no major damage or casualties.
But it is clearly a worrying development for the mission.
Security around the embassy's two compounds was supposedly greatly tightened by the Iranian authorities after at least three shooting attacks in September.
The obvious question is how the firebomb attempt could be carried out without the culprit being stopped.
The suspicion and fear is that it may have been the work of a lone hardliner, inspired by the attack on British targets in Istanbul just the day before.
Those attacks have won no apparent sympathy here though.
Just a few hours before the firebomb was thrown, tens of thousands of mainly hardline Iranians had marched through the streets of Tehran to mark Jerusalem Day.
All of those I questioned condemned the Istanbul bombings.
There is no other immediately obvious explanation for the attack on the embassy.
A major irritant in relations between the UK and Iran, the extradition case brought by Argentina for a former Iranian ambassador living in Britain, was resolved last week.
The man has since returned to Tehran.