"I wouldn't rule out the possibility that could include the UK, but we are already operating at a high level of Iran-generated threat.
"For the most part, the activity we've seen backed by Iran in the UK for the last 18 months or so has been targeted at the regime's own internal enemies, dissidents, Farsi media organisations. Not exclusively, but predominantly.
"Clearly one of the things on our minds is, might the Iranian targeting intent shift in response to events elsewhere."
MI5 Director General Ken McCallum (Photo: Alamy)
McCallum, who joined his counterparts from the Five Eyes intelligence services - the UK, US, Canada, Australia and New Zealand at a summit in California, also said MI5 was paying "very close attention" to events in the Middle East.
He added: "There clearly is the possibility that profound events in the Middle East will either generate more volume of UK threat or changes in shape in terms of what is being targeted, in terms of how people are taking inspiration.
"Terrorists can draw inspiration not just from things they see happening inside the UK but things they see happening in the Middle East or on the continent or elsewhere.
"So, we would be silly not to be paying very close attention, and we are."
The MI5 chief had previously warned Iran was the state which "most frequently crosses into terrorism."
McCallum, an MI5 veteran appointed to the top job in March 2020, also revealed the national terror threat level - which indicates the likelihood of a terrorist attack in the UK - was under constant review.
He added: "The threat level, set by the independent Joint Terrorism Analysis Centre (JTAC), sits at 'substantial', which translates as 'an attack is likely'. So that is already the level at which we sit.
Hamas launched a terrorist attack on Israel earlier this month (Photo: Flash 90)
"Clearly there are other levels and the analysts within JTAC will be keeping under review whether that remains the appropriate level."
Pressed on whether it would need to be increased, he said: "I wouldn't want to speculate but if the evidence is there to justify a rise, then of course, that's what those analysts will recommend.
"I spent large parts of my career seeking to detect terrorists plotting in time to get ahead of it. So I am, in effect, always concerned about those risks. But yes, naturally the events of the last two weeks have sharpened that concern still further."
It comes just weeks after Foreign Secretary James Cleverly ruled out the UK Government proscribing Iran’s Islamic Revolutionary Guard Corps (IRGC) as a terrorist group despite its links to Hamas.
