A total of 43 late-stage plots to commit “mass murder” in the UK have been foiled since 2017, he said, involving firearms and explosives.
Mr McCallum said the security service’s counter-terrorism work still mostly involved Islamic extremism, followed by “extreme right-wing terrorism”.
But he added there was a “dizzying range of beliefs and ideologies” MI5 had to deal with.
“The first 20 years of my career here were crammed full of terrorist threats. We now face those alongside state-backed assassination and sabotage plots, against the backdrop of a major European land war,” he said.
In a briefing at MI5’s counter-terrorism operations centre in London, Mr McCallum said in the past year, the number of state-threat investigations by MI5 had increased by 48%.
Mr McCallum accused Russia of “arson, sabotage and more dangerous actions” in the UK.
He said more than 750 Russian diplomats had been expelled from Europe since Russia’s invasion of Ukraine, saying “a great majority of them” were spies.
This affected the Russian intelligence services’ capability, McCallum explained, and added that diplomatic visas had been denied to those Britain and allies considered Russian spies.
Russian state actors turned to proxies, such as private intelligence operatives, to do “their dirty work”, but this impacted the professionalism of their operations and made them easier to disrupt.
He also told the briefing that MI5 and police had responded to 20 Iran-backed plots since 2022, which presented “potentially lethal threats to British citizens and UK residents”.
He said targeted counter-terrorism work remained split between “75% Islamist extremism, 25% extreme right-wing terrorism”.
Mr McCallum expressed concern over the number of young people being drawn into online extremism.
About 13% of those investigated for involvement in terrorism were under 18 – a threefold increase in the last three years, he added.
He said other issues MI5 encountered were exacerbated by the internet, and a high proportion of the threat was made up by “lone individuals indoctrinated online”.
“In dark corners of the internet, talk is cheap. Sorting the real plotters from armchair extremists is an exacting task,” he told reporters.
“Anonymous online connections are often inconsequential, but a minority lead to deadly, real world actions.”
He spoke of the difficulty the security service faces while dealing with “volatile, would-be terrorists with only a tenuous grip of ideologies”.
