MI5 is devoting more resources to countering state threats than at any point during the Cold War, the head of the intelligence service has said.

Sir Ken McCallum, speaking at his annual address in London, said threats from foreign adversaries had quadrupled over the last decade. He named Russia as "aggressor number one" and warned of a sharp rise in Iran-linked aggression targeting Jewish communities and military sites in the United Kingdom.

Sir Ken said Russia was waging hybrid warfare at a time when artificial intelligence was "supercharging the cyber-security contest between attackers and defenders." He described Russia's progress in its full-scale invasion of Ukraine as "minimal and horrifically costly," adding that Moscow had stopped short of conventional conflict with NATO and was instead attempting to unsettle European countries through non-military means.

Hybrid attacks typically refer to methods such as cyber-attacks, sabotage and disinformation campaigns, often carried out through proxies. Russia has consistently denied involvement in such operations despite what Sir Ken described as growing evidence to the contrary.

The MI5 director general said the service's success in identifying and expelling Russian intelligence officers had left the Kremlin reliant on criminal proxies. "With its traditional covert footprint eviscerated by almost 1,000 expulsions since the invasion, the Kremlin's global toolkit is weak," he said. "So we see them scrambling to pay criminals to do their bidding."

He added: "These are not the actions of a great power with the strategic upper hand."

Speaking to reporters after the speech, Sir Ken was asked about recent comments by Prime Minister Andy Burnham, who said Russia had interfered in the 2016 Brexit referendum. Sir Ken said it was "not new news" that Moscow had made such attempts, but drew a distinction between "interference attempted" and "outcome achieved."

He acknowledged it was a matter of public record that Russian media and "so-called internet trolls and bot farms" had sought to influence the Brexit debate, while stressing that the result of the referendum was "not in dispute."

He expressed concern that the ongoing public debate risked overstating the impact of Russian influence operations. "I am a little bit anxious that we don't play into our adversaries' hands by making these trolls look 10ft tall," he told reporters.

On Iran, Sir Ken said the country was expanding a hybrid campaign inside the United Kingdom. He said there had been 50 arrests in the past year linked to Iranian activities, with much of the aggression directed at Jewish communities and, in his words, "harder targets, like military sites."

Although he did not name RAF Fairford directly during his speech, he told reporters afterwards that the incident there was "not unique," adding: "We have seen Iranian interest beyond RAF Fairford in the UK." Seven people have been arrested and bailed by counter-terrorism police in connection with the airbase in Gloucestershire, where three suspicious vans were observed parked nearby. Burnham has said the incident was "almost certainly directed by Iran's Islamic Revolutionary Guard."

Sir Ken said in his speech: "What I can say is that those who seek to cause mayhem on British streets - whether hired thugs here or their paymasters overseas - will meet an unflinching response."

He also noted that the National Security Act, a 2023 law targeting individuals working against UK interests on behalf of foreign powers, was in heavy use and had been applied to threats from China as well as Iran and Russia.

On the threat posed by artificial intelligence, Sir Ken warned that the technology could be exploited to produce propaganda and radicalising content tailored to individuals. "We've caught terrorists seeking to exploit AI to improve their reconnaissance and attack methodologies, with AI-enabled drones adding a new twist," he said.

He identified the misuse of AI to develop bioweapons as the most serious emerging risk, though he noted the threat was not yet fully realised. He also warned of rising dangers from extreme right-wing terrorism.

Sir Ken said that countering terrorism and countering state threats were now operating at "pretty much of equal scale," describing the shift as significant. MI5, he said, was now committing more resource to state threats than at any time since the Cold War.