UK's Foreign Secretary Dominic Raab says the brief arrest of Envoy to Iran Rob Macaire, without grounds or explanation, is "a flagrant violation of international law".
He noted that if Tehran does not take steps towards de-escalation, the nation may face political and economic isolation.
"The arrest of our Ambassador in Tehran without grounds or explanation is a flagrant violation of international law. The Iranian government is at a cross-roads moment. It can continue its march towards pariah status with all the political and economic isolation that entails, or take steps to deescalate tensions and engage in a diplomatic path forwards", the official statement said.
The US Department of State also called for a formal apology to the United Kingdom on the part of Tehran.
Ambassador Macaire was arrested in Tehran during the protests to honor those killed in the fatal Ukrainian plane crash in Iran. The demonstrations, however, resulted in a rally demanding the resignation and prosecution of those responsible for the incident. Police dispersed the protesters who demonstratively tore the portrait of slain Gen. Qasem Soleimani killed in the US targeted attack on 3 January.
"The UK ambassador in Tehran, who attended the rally and was involved in organising, inciting, and also leading some radical actions, was detained for several hours", Tasnim News Agency reported, citing a source.
The outlet added that Macaire would be summoned to Iranian Foreign Ministry on Sunday.
On Saturday, Iran admitted the Ukrainian Boeing 737 was unintentionally downed by its military, taking full responsibility for the crash and expressing its condolences to the families of the victims and all nations involved.
According to the commander of the Aerospace Force of the Islamic Revolutionary Guard Corps, Amir Ali Hajizadeh, Iran's military mistook the jet for a cruise missile and shot it down by mistake. Iran had previously insisted the crash that killed all 176 people on board occurred due to a technical error.