The Foreign Office expressed its "outrage" over the decision of Iranian authorities to put on trial a worker at the British Embassy in Tehran.
A spokeswoman confirmed that Hossein Rassam was pictured among the accused at a mass trial.
She said: "This is completely unacceptable and directly contradicts assurances we had been given repeatedly by senior Iranian officials."
The spokeswoman added: "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."
Mr Rassam, the Embassy's chief political analyst, was the last of nine British Embassy workers to be freed following their arrest in June.
