According to diplomats, three to four shots were fired at the embassy overnight.
Last Wednesday, the embassy was temporarily closed after shots were fired from a passing motorbike.
Although no one was hurt on either occasion, the shootings underlined the tense relations between the two countries over Britain’s arrest of a former Iranian diplomat in connection with the 1994 bombing of a Jewish centre in Argentina that killed 85 people.
Iran has said the case is politically motivated and called for the swift release of Hadi Soleimanpour, who was Iran's ambassador to Argentina at the time of the 1994 bombing. He was studying in Britain when arrested last month.
Iranian officials said Iran's ambassador to Britain, who came to Tehran for consultations over the dispute, returned to his post on Sunday. Some newspapers have called for Iran to expel the British ambassador.
"We've protested strongly to the Iranian Ministry of Foreign Affairs that security precautions taken by the Iranian authorities after last week's shooting did not prevent this further incident," a Foreign Office spokesman said.
"We consider it a further failure of our hosts' duty to protect the embassy under the Vienna convention," the spokesman said.
"The embassy has called once again for an urgent
investigation. We want to see those responsible brought to justice."
The Iranian authorities have said they were carrying out an investigation into last week's shooting.