Foreign Secretary David Miliband condemned Iran's "intimidation" tactics today amid reports that eight British embassy staff have been arrested.
Mr Miliband insisted the action against the UK's "hard-working" local employees in Tehran was "quite unacceptable".
"This is harassment and intimidation of a kind which is completely unacceptable," he said.
Mr Miliband indicated that "strong action" would follow if the detentions did not stop.
The comments, at a meeting of foreign ministers in Corfu, came as the diplomatic spat between the UK and Iran intensified in the wake of contested elections.
Tehran has accused the UK and the US of seeking to interfere in its internal affairs after they criticised the heavy-handed reaction to protests by opposition supporters.
The row led to the tit-for-tat expulsion of diplomats from London and Iran.
Mr Miliband said today: "The idea that the British Embassy is somehow behind the demonstrations and protests that have been taking place in Tehran... is wholly without foundation."
Mr Miliband said he believed nine local staff had been detained, although some had since been released.
"We have protested in strong terms, directly to the Iranian authorities, about the arrests that took place yesterday," he said.
"I think it is very important that I discuss this with my EU colleagues later on this afternoon. All European countries have made clear that they want to stand together in standing up for the diplomatic principles that are important for our diplomatic activity all over the world.
"At the moment our top priority is the position of our locally-engaged staff who we want to see released unharmed and back to work."Reuse content