MI5 Director-General Ken McCallum used his annual speech on Tuesday to say there had been a major shift in Tehran's calculus this year and that there were "Iranian ambitions to come at harder targets, like military sites".
His warning follows incidents at two Royal Air Force bases. Prime Minister Andy Burnham said Iran's Islamic Revolutionary Guard Corps (IRGC) was "almost certainly" behind a suspected plot against RAF Fairford, an airbase used by US bombers to carry out strikes on Iran. The United States withdrew its B-1 Lancer and B-52 bombers from the Gloucestershire facility on October 4 after a security breach.
British police declared a major incident after three "suspicious" vans drove close to RAF Fairford on September 27. Five British men in their 20s were arrested on suspicion of offences under the Explosives Act and released on bail. Police found a supply of petrol in three vans linked to the suspects, but no explosives. Two others, including a British-Iranian national, were arrested and later released. None of the men have been charged.
Two Latvian men were also arrested for trespassing at RAF Molesworth, a base used by US and NATO intelligence operations, and released on bail.
Foreign Secretary Ed Miliband said on Monday that the government was sanctioning two Iranians for their "role in facilitating or engaging in hostile activity on behalf of the Iranian regime". Iran has denied any involvement in the incidents, with Iranian media framing the British accusations as "false flag" operations.