The Foreign and Commonwealth Office has stated:
"Speaking in the House of Commons on Wednesday, Foreign Secretary William Hague described the attack on the British Embassy in Iran on 29 November as "a breach of international responsibilities of which any nation should be ashamed." The Foreign Secretary said that the attack, in which Embassy buildings were vandalised and looted by demonstrators, was a grave violation of the Vienna Convention which states that a host state is required to protect the premises of a diplomatic mission against any intrusion, damage or disturbance. The Foreign Secretary expressed his gratitude for the strong statements of concern and support from the United States, the European Union, Germany, Poland, Russia, China and many other nations.
The British Embassy in Tehran has now been closed. All Iranian diplomatic staff have been ordered to leave the United Kingdom."
Further details of the events at the embassy in Tehran are at The Guardian 29th November - "Storming of British Embassy in Tehran worsens bilateral relations."
The Foreign Secretary is of course absolutely right in describing the event as a grave violation of the Vienna Convention on Diplomatic Relations 1961. Article 22 states: