Foreign Secretary William Hague has said Iran faces “serious consequences” after the British Embassy in Tehran was stormed by hardline students.

Rocks and petrol bombs were thrown, while documents were removed from offices and burnt, Iranian news agencies reported.

The break-in came after hundreds of students gathered outside the embassy as part of a protest against what they described as London’s “hostile policies” against Iran.

Protesters reportedly set fire to the Union Flag as they chanted “Down with Britain”, “Down with America” and “Down with Israel”. There was extensive damage to property.

The protests later ended and the students left the main compound in central Tehran – and a second embassy compound in the north of the capital – after being given an “ultimatum” by security forces, local reports said.

The raid was condemned by Mr Hague, who said it marked a “serious failure” by the Iranian government to take measures to protect British Embassy compounds.

He said all embassy staff were accounted for and that he had called on Iran to ensure the safety of UK personnel, return any stolen property and secure the compounds immediately.”

He will reveal “other, further, and serious consequences” on Wednesday.

The attack was also criticised by the European Union and the White House, which urged Iran to prosecute the offenders.

A human chain outside the Uranium Conversion Facility earlier this month

Iran’s parliament on Sunday voted to reduce diplomatic relations with the UK following London’s support for increased sanctions on Iran.

Earlier Mr Hague told MPs in the House of Commons that the UK would “respond robustly” if the authorities in Iran expelled its ambassador from Tehran.

He said: “It will do nothing to repair Iran’s international reputation.”

Iran’s foreign ministry released a statement saying it “regrets the protests that led to some unacceptable behaviours… We respect and we are committed to international regulations on the immunity and safety of diplomats and diplomatic places”.

Relations between the West and Iran have been deteriorating in recent months amid fears that Iran is seeking to develop nuclear weapons. 

Iran insists its nuclear programme is to provide electricity for civilian use.

Iran last week said it had arrested 12 CIA agents, accusing them of planning to strike at Iranian interests.