British embassy worker Hossein Rassam is seen among others at the mass trial in Iran
A decision by the Iranian authorities to put a British Embassy worker on trial in Tehran has been condemned as an "outrage" by the Foreign Office.
It confirmed Iranian Hossein Rassam - the embassy's chief political analyst - was among the accused at a mass trial which began on Saturday morning.
Mr Rassam is accused of spying and inciting unrest over President Mahmoud Ahmadinejad's re-election in June.
Nine embassy employees were arrested but he is the only one to be charged.
A Foreign Office spokeswoman said the British ambassador in Tehran was seeking clarification on "this latest outrage".
"This is completely unacceptable and directly contradicts assurances we had been given repeatedly by senior Iranian officials," she said.
"We deplore these trials and the so-called confessions of prisoners who have been denied their basic human rights."
Mr Rassam is accused of "spying for foreigners", Iranian state media quoted the judge as saying.
Prosecutors accuse him of monitoring the riots on the ground along with two UK diplomats who have since been expelled.
The prosecutors say he was asked to meet representatives of political groups, ethnic and religious minorities and student groups, and inform London about the riots, state media reported.
State media said he had apologised for "the clear violations he had committed".
The Foreign Office said it was awaiting news from the British ambassador before commenting on the reports of evidence.
Mr Rassam was held after the protests with eight other embassy colleagues and later released on bail, a move welcomed by the Foreign Secretary David Miliband.
At the time he said: "The detention of embassy staff was completely unjustified: I am confident that none of them were involved in any improper behaviour."
An Iranian employee of the French embassy, Nazak Ashfar, was also in the dock, state media said, as was French teacher Clotilde Reiss.
The 24-year-old was stopped on 1 July as she prepared to leave Tehran, having spent five months as a university teaching assistant.
She is accused of "collecting information and provoking rioters", IRNA news agency said.
France says the allegations against her are "absolutely baseless"
Rights groups say that hundreds of people including reformist politicians, journalists, activists and lawyers have been detained in Iran since the election.
Violent street protests broke out after President Mahmoud Ahmadinejad was re-elected in the 12 June vote.
Iran has repeatedly accused foreign powers - especially Britain and the US - of stoking the demonstrations.
Opposition candidate Mir Hossein Mousavi says the vote was rigged in favour of Mr Ahmadinejad.
The president and Iran's main election body, the Council of Guardians, have rejected the charge.