"The UK government is clear that protecting national security remains our top priority and Iran must be held accountable for its actions," the Foreign Office said in a statement.

It added that Iran's ambassador "Seyed Ali Mousavi was summoned in response to three Iranian nationals charged under the National Security Act."

Earlier Monday, Tehran summoned a British envoy in the Iranian capital to protest the arrests.

Three Iranian men appeared in a London court on Saturday charged with spying for the Islamic republic's intelligence services.

They were arrested on May 3 and identified as Mostafa Sepahvand, 39, Farhad Javadi Manesh, 44, and Shapoor Qalehali Khani Noori, 55, all living in London.

The British Home Office said they were irregular migrants who arrived by small boat or other means, such as hidden in a vehicle, between 2016 and 2022.

The BBC reported that the three were allegedly plotting to attack UK-based journalists working for the Farsi-language Iran International television news network which has been labelled a "terror" organisation by Tehran.

The alleged spying took place from August 2024 to February 2025, according to UK police.

A fourth man was arrested on May 9 as part of the investigation, but has now been released without charge, the police said in a statement.

Five Iranians were also arrested on May 3 in a separate investigation.

Four of the men -- who had been held on suspicion of preparing a terrorist act -- were released, although the investigation "remains active and is ongoing", police said.

The fifth was earlier bailed until an unspecified date in May.