Britain is confronting a mounting threat from hostile powers including Russia, Iran and China, the UK’s domestic spy chief warned on Thursday.

Delivering his annual address at MI5’s London headquarters, Director General Ken McCallum revealed that there has been a 35% rise in the number of individuals under investigation for threats from hostile states.

“In 2025, MI5 is contending with more volume and more variety of threat from terrorists and state actors than I’ve ever seen,” he said.

The scale of the danger marks what McCallum described as a “new era” for the agency, driving the “biggest shift in MI5’s mission since 9/11.”

Referencing activities linked to Russia, Iran and China, McCallum said that such hostile nations are resorting more frequently to the “ugly methods MI5 is more used to seeing in our terrorism casework.”

He disclosed that the agency has thwarted a “stream of surveillance plots with hostile intent” emanating from Russia.

According to McCallum, would-be “proxy” actors are treated as expendable by Russia as he cautioned potential operatives that “when you’re caught, you’ll be abandoned.”

He added that Russian online trolls continue to probe for “fissures” within British society, though he noted that these efforts have largely failed to gain traction.

It is the latest in a long line of recriminations by the UK against Moscow, which denies participating in any wrongdoing in the country.

In September, Scotland Yard said that three people had been arrested in Essex on suspicion of spying for Russia, as the head of the UK’s counter-terrorism command warned that “we’re seeing an increasing number of persons who we would describe as ‘proxies’ being recruited by foreign intelligence services.”

In May, six Bulgarians were convicted of belonging to a Russian espionage cell described as something out of a “spy novel” complete with forged documents, hidden cameras, a spy love triangle and secret online chats about potential abductions and honeytraps.

Authorities say the group’s activities rank among the “largest and most complex” enemy operations ever uncovered on British soil.

Five men have also been convicted over an arson attack in March last year which saw a warehouse containing vital communications equipment bound for Ukraine set alight, causing £1 million ($1.3 million) of damage.

Britain said that those responsible had acted as “arsonists for hire” for Russia’s Wagner mercenary group.

“With our partners across Europe, MI5 will keep detecting those who take orders from Russian thugs,” McCallum said.

“And we will keep following the trails back to those giving the orders, who imagine they’re anonymous and unfindable behind their screens. They’re not.”

McCallum also underscored the threat from China, accusing Beijing of cyber espionage, enticing academics to China, meddling in public institutions and intimidating pro-democracy dissidents on British soil.

Turning to Iran, he said the regime is “desperately” trying to silence its critics overseas.

“MI5 will keep doing what the public would expect of us, preventing, detecting and disrupting activity of national security concern,” the MI5 chief concluded on Thursday.

It comes after MI5 issued security guidance to UK lawmakers on Monday warning them that spies from Russia, China and Iran may target MPs and their staff using “long-term relationship building, blackmail, phishing attacks and disinformation activity.”