The dispute over the arrest of Britain’s ambassador to Tehran has escalated as the Foreign Office summoned the Iranian ambassador in order to protest about the episode.

“This was an unacceptable breach of the Vienna convention and it needs to be investigated,” a spokesperson for the prime minister said. “We are seeking full assurances from the Iranian government that this will never happen again. The FCO has summoned the Iranian ambassador today to convey our strong objections.”

There had been signs that Iran’s foreign ministry wanted to calm the situation. The country’s ambassador to the UK, Hamid Baeidinejad, had tweeted that Rob Macaire was released “the moment the police were informed of his identity.” “Misinformation is a major source of misunderstanding and detrimental to tranquility,” he said.

Demonstrations outside the UK’s embassy, however, and continued demands from Iranian hardliners that the embassy be closed have prompted the Foreign Office to tell Baeidinejad personally that the arrest was in breach of diplomatic convention.

Iranian news agencies have published what appear to be police videos showing Macaire at what he says was a vigil for Iranians killed when a Ukrainian plane was shot down last week. Four Britons were among the 176 people killed in the crash.

The film, titled “UK ambassador attending illegal protest”, did not appear to show Macaire doing anything to agitate or coordinate the protest, as some Iranian officials have alleged.

Iran’s foreign ministry on Monday warned Britain not to make any “new mistakes”, in an apparent reference to Macaire’s presence at the vigil.

“Any new mistake of Britain will be severely confronted by Iran and London will be responsible for that. The presence of the UK envoy is against diplomatic norms... Threatening Iran with fresh sanctions over that will cause tension between Tehran and London,” the statement said, according to state TV.

The UK foreign secretary, Dominic Raab, told MPs “the ambassador attended a public vigil to pay his respects to the victims of flight 752. He left shortly after arrival when there were signs that the vigil might turn into a protest.

“He was not attending or recording a political protest or demonstration. His arrest later that day, without grounds or explanation, was a flagrant violation of international law.”

Raab said Iranian officials had to show some accountability for the attack on the plane, which was sanctioned by the Revolutionary Guards. “There must be a full, thorough transparent investigation with an international component,” he said..

He also said he would be discussing the matter in London on Thursday with fellow foreign ministers from countries whose nationals were killed in the attack. The meeting will include the issue of compensation.

Iranian officials denied reports that the Emir of Qatar had offered to establish a multibillion pound compensation fund during a visit to Tehran at the weekend.



