Foreign minister William Hague said Britain took the storming of its embassy in Tehran on Tuesday "extremely seriously" and warned there would be "further and serious consequences" for Iran.
Hague said the assault on the embassy and another diplomatic compound in Tehran by young men chanting "Death to Britain" was a "grave breach" of the Vienna Convention which requires host countries to protect diplomats.
"The United Kingdom takes this irresponsible action extremely seriously," he said.
"We hold the Iranian government responsible for its failure to take adequate measures to protect our embassy, as it is required to do.
"I spoke to the Iranian foreign minister this afternoon, to protest in the strongest terms about these events and to demand immediate steps to ensure the safety of our staff and of both embassy compounds."
Hague said in a statement that although the Iranian foreign minister had apologised, "this remains a very serious failure by the Iranian government".
"Clearly there will be other, further, and serious consequences," Hague said, adding that he would make a statement to the British parliament on Wednesday.
British Prime Minister David Cameron chaired a meeting of the government's crisis committee in response to the storming, which has raised tensions with the West over Iran's contested nuclear programme.
France, the European Union and Russia -- Iran's closest ally -- have condemned the attack on the British facilities.
Six British diplomats held for more than two hours by hundreds of protesters inside a British diplomatic compound in the north of Tehran were freed when police intervened.
Hague said: "On our latest information it now appears that all our staff and their dependants are accounted for."
But he added British officials were still "urgently establishing" if all the locally hired security staff were safe.
Hague also repeated advice given earlier in the day that the "small number" of British nationals in Iran should stay indoors and await further instructions.
Dozens of protesters were shown on Iranian state television throwing stones at embassy windows, breaking them, and one was seen climbing the wall of the compound carrying a looted portrait of Britain's Queen Elizabeth II.
The attack comes amid heightened tensions following Iran's decision to pass a law to expel the British ambassador to Tehran in retaliation for new British sanctions that cut off all ties with Iran's financial sector.
The sanctions were part of a coordinated raft of measures announced on November 14 by Britain, the United States and Canada to increase pressure on Iran to halt its nuclear programme.
Iran insists it is not developing a nuclear weapon and that the programme is purely for energy purposes.