Miliband said he was "deeply concerned" about staff being detained, stressing his confidence that they had not engaged in illegal activities despite Iranian allegations they had instigated post-election violence.
"We have noted the remarks by Ayatollah (Ahmad) Jannati suggesting that some of our local staff in Iran may face trial," Miliband said in a statement.
"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."
Jannati, the head of Iran's powerful Guardians Council, said at Friday prayers that some local British embassy staff would be put on trial for their alleged role in stoking violence after the June 12 presidential vote.
A total of nine local staff at the British embassy in Tehran were initially arrested late last month but the British government said seven have been released, while Iranian state television has said only one remains in custody.
Foreign Office permanent under-secretary Peter Ricketts summoned Iranian ambassador Rasoul Movahedian in London and "stressed the need for urgent clarification" of the issue.
"He (Ricketts) reiterated the need for the detained British Embassy staff in Tehran to be released as soon as possible and without charge," a Foreign Office statement said.
The calling-in of the ambassador was part of a coordinated protest by EU countries over the detention of the British embassy staff, a move welcomed by Miliband.
An EU diplomat said: "We can confirm that one of the actions agreed at yesterday's political directors' meeting was that EU members should as soon as possible summon the Iranian ambassadors across the EU to demand the immediate release of embassy personnel."
Tehran has already expelled two British diplomats from Iran in actions reciprocated by London, over Iranian allegations that Britain stirred unrest that erupted over the disputed re-election of President Mahmoud Ahmadinejad.
The president's rivals say his victory was fraudulent and marred by widespread irregularities.
A Foreign Office spokeswoman said: "Allegations that our staff were fomenting unrest are wholly without foundation."
