"If this action continues and we are forced to act, we will act together with our European partners," Brown said at a joint news conference with French President Nicolas Sarkozy in the French Alps.
Downing Street confirmed earlier that Iran had released an eighth local staff member of the British embassy in Tehran, leaving one still in detention.
Iran accused the embassy employees of having instigated riots in the unrest that erupted over the re-election of President Mahmoud Ahmadinejad, which his rivals said was fraudulent and marred by widespread irregularities.