LONDON: Britain said the trial Saturday of one of its Iranian embassy workers over the protests in Tehran was "completely unacceptable" and was preparing to set out its response to the "outrage".

British embassy local staffer Hossein Rassam was in the dock in a Tehran court alongside others detained during protests over the re-election of President Mahmoud Ahmadinejad.

Rassam has been accused of spying for foreigners, the official IRNA news agency said.

"We can confirm that Hossein Rassam is pictured among the accused at the mass trial which began in Tehran this morning," a Foreign Office spokeswoman said.

"This is completely unacceptable and directly contradicts assurances we had been given repeatedly by senior Iranian officials.

"We deplore these trials and the so-called confessions of prisoners who have been denied their basic human rights.

"Our ambassador in Tehran has demanded early clarification of the position from the Iranian authorities. We will then decide on how to respond to this latest outrage."

Senior Iranian figures accused Britain of orchestrating the opposition demonstrations following the June 12 election, which were put down by security forces amid the worst turmoil in the Islamic republic's 30-year history.

Tehran expelled two British diplomats and London retaliated with tit-for-tat expulsions. Ties between the countries were further strained when Iranian authorities arrested nine British embassy staff, although all have been released.

Britain's ambassador to Iran attended the Ahmadinejad's inauguration on Wednesday because "hard-headed diplomacy" is required in dealings with the country, the Foreign Office said.

Although ambassador Simon Gass was present at the ceremony in Tehran, Britain did not send a message of congratulations.

Britain had made its position clear about the "abuse of human rights" that followed Ahmadinejad's victory in an election which opposition supporters claim was rigged, the Foreign Office said.