A British embassy employee is to stand trial in Tehran for "acting against national security" — a dramatic escalation in Iran's campaign to blame Britain for protests against disputed election results.

The man, a 44-year-old Iranian who is the British embassy's chief political analyst, was arrested on Saturday and has been formally charged at Tehran's Evin Prison, his lawyer, Abdolsamad Khorramshahi, said. "Apparently he will be put on trial. We have prepared and submitted the defence documents and I have to see the judge next week."

It was is not clear whether any other embassy staff will face prosecution. A senior cleric claimed that some had "confessed" to playing a role in the protest movement. The staging of political trials is likely to lead to a breach in relations not only with Britain, but also with the European Union. Iranian ambassadors were summoned to foreign ministries in capitals across Europe in a coordinated rebuke .

The analyst is one of two Iranian staff of the British embassy still being held for allegedly playing a role in the protests that followed the official victory in last month's presidential elections of President Mahmoud Ahmadinejad. Seven other staff members have been released from detention in recent days but warned they could face further legal proceedings. Earlier in the day Ayatollah Ahmad Jannati, head of a highly influential body of clerics, the Guardian Council, claimed that some embassy employees had "confessed" to playing a role in post-election demonstrations, and would be prosecuted.

The cleric is close to Iran's Supreme Leader but he is not in charge of the judiciary, so British officials insist that trials were still not inevitable even though charges had been laid.

"Acting against national security" is a vague charge often brought against political activists and is not known to carry any fixed sentence. The charge was levelled against three US-Iranian academics detained in 2007 while visiting Iran. All three were subsequently released.

The foreign secretary, David Miliband, said: "We are confident that our staff have not engaged in any improper or illegal behaviour." Miliband added that Britain was "deeply concerned" about the fate of the two embassy staff. However, their plight presents British officials with a dilemma. If they provide too much overt help it will strengthen Tehran's depiction of them as "foreign agents".

Britain withdrew a request for European states to pull ambassadors from Tehran after the Iranian government released some embassy staff on Wednesday, but London is likely to look for stronger action if the trials proceed. European officials at meetings at Stockholm and Brussels said the option of withdrawing ambassadors remained on the table. The Europeans also discussed the possible penalty of blacklisting regime officials by temporarily blocking visa applications to travel to the EU.

"We view this not just as an attack on Britain, but as an attack on the entire European Union," said a European official.

Carl Bildt, the foreign minister of Sweden, which took over the EU presidency this week, said it was not acceptable to file charges against British embassy staff. But the threat of charges being pressed against embassy employees, followed by trials, looked calculated to call Europe's bluff and to gauge how the EU might respond.

"Our solidarity … is total. Now it is up to the British to tell us what they need," said the French president, Nicolas Sarkozy. "France has always wanted to strengthen the sanctions so that the Iranian leaders really understand that the path they have chosen will be a dead end."

France has taken a tough line on the nuclear dispute with Iran over the last six years, while Germany and Italy, with billions of euros in trade at stake with Iran, have been less keen on sanctions.

Today's meeting in Brussels agreed on a common protest to the ambassadors. Officials said the crisis could be taken to a summit of G8 leaders in Italy next week, although the Italians are seen as the least supportive of strong action against Iran.