In his annual speech on wide-ranging threats to Britain, MI5 Director General Ken McCallum also said the greatest concern about the misuse of AI was terrorists being able to develop bioweapons or even create new pathogens.
McCallum said state threats had quadrupled over the last decade, and his organisation now devoted more resources to the issue than during the Cold War.
On Monday, British Prime Minister Andy Burnham said Tehran was "highly likely" behind a suspected plot against RAF Fairford, an air base used by US bombers to carry out strikes on Iran. It prompted a security alert and led to US President Trump withdrawing planes from the air base.
Tehran has rejected accusations of involvement in the Fairford incident or any other accusations of malign activity in Britain where the authorities have for a number of years accused it of being involved in plots and attempted assassinations against dissidents and Israeli and Jewish targets.
Russia also denies involvement in hostile acts in Britain and other Western countries, but McCallum said it remained the "aggressor number one".
Russia's embassy in London said in a statement: "Attributing every cyber incident, drone mishap and criminal case in Europe to Moscow is convenient, particularly when no evidence is offered."
It said promoting a "supposed Russian threat" was being used to justify expanded security powers and militarisation as well as to deflect from domestic problems.
"However, repeating baseless accusations does not turn them into facts," it added.
McCallum said he could not speak about the Fairford incident because of an ongoing investigation, but he said there had been a major shift this year in "Iranian ambitions to come at harder targets, like military sites".
"The RAF Fairford incident ... is not unique. There have been other examples, not just in the UK but around the world," he said.
McCallum said over the past year Iran-linked operations had led to more than 50 arrests.
Asked if Iran had the capability to launch complex attacks perhaps using explosive drones, McCallum said it was limited by the sophistication of the criminals and proxies it used for operations in Britain.
"Given what we've actually seen in practice in the UK in recent years, that has not been anywhere near (that) level of capability," he said.
"That said, clearly it is the case that in countries closer to its own homeland, the IRGC (Islamic Revolutionary Guard Corps) has quite deep experience of using more conventional and hybrid technology in a more aggressive and sophisticated way."
McCallum echoed comments from other European spy agencies that Russia was using tactics just short of conventional conflict, citing the attempted drone attack on a cargo plane at a German airport and a cyber attack on Polish energy infrastructure.
"It's no accident that Russian aggressors are trying to unsettle other European homelands at a time when their military progress in Ukraine is minimal and horrifically costly."
