Abdolsamad Khorramshahi said his client was being held in Evin prison and would probably stand trial soon.
The lawyer identified his client as Hossein Rassam, the embassy's chief political analyst, and said he had not yet been able to meet him or
see the text of the indictment.
In June Iran arrested nine local members of the British embassy staff, saying they were involved in post-election unrest.
Iranian news agencies said all but one had since been released.
Ayatollah Ahmad Jannati, head of Iran's powerful Guardian Council, said staff had made confessions but did not say how many faced trial or on what charges.
"In these events, their embassy had a presence," he said. "Some people were arrested. Well, inevitably, they will be put on trial," he said last week.
Meanwhile, Foreign Secretary David Miliband said he was seeking urgent talks with his counterpart in Tehran.
Miliband said: "We have noted the remarks by Ayatollah Jannati suggesting that some of our local staff in Iran may face trial.
"We are urgently seeking clarification from the appropriate Iranian authorities. I intend to speak to foreign minister (Manouchehr] Mottaki.
"We are confident that our staff have not engaged in any improper or illegal behaviour. We remain deeply concerned about the two members of our staff who remain in detention in Iran."
Embassy staff were arrested last weekend amid rapidly deteriorating relations with Iran since the disputed re-election of president Mahmoud Ahmadinejad.
Shadow foreign secretary William Hague condemned the prospect of a "show trial" and demanded a strong response from other European Union countries.
A top aide to Iran's supreme leader called the country's main opposition figure a US agent and said yesterday that he should be tried for crimes against the nation.
