Britain is pressing Iran's Foreign Ministry to help secure the release of four local employees of the British embassy who are being held by Iranian authorities.
Nine Iranian employees of the embassy have been accused of stirring up street protests after a disputed presidential election on 12 June. Britain rejects the accusation.
Five personnel have been released, but four are still being held.
Foreign Secretary David Miliband said on Tuesday that their continued detention was "unacceptable harassment".
Official results showed President Mahmoud Ahmadinejad won the election by a landslide. His main moderate challenger, former prime minister Mirhossein Mousavi, says the vote was rigged.
State media say about 20 people were killed.