Russia is intent on causing “mayhem” on UK streets, and Iranian-backed plots posing “lethal threats” to British people are ramping up at an “unprecedented pace and scale”, the boss of MI5 warned.

Director general Ken McCallum issued the stark warnings less than two years after he laid bare the “very real threat” posed by aggression from hostile states.

In a wide-ranging speech, he also highlighted the “worsening threat from al Qaida and in particular from Islamic State”, which he said had “resumed efforts to export terrorism”, and said MI5 was “powerfully alive” to the risk that tensions in the Middle East posed to terrorist activity in the UK.

Mr McCallum told reporters the rise in the number of children being investigated for terrorism in the UK by the security agency is “staggering” and warned of “canny online memes” drawing them into extreme right-wing ideologies.

Speaking at the Counter Terrorism Operations Centre in west London on Tuesday, he said: “MI5 has one hell of a job on its hands. The first twenty years of my career here were crammed full of terrorist threats.

“We now face those alongside state-backed sabotage and assassination plots, against the backdrop of a major European land war.”

In the last year, the number of state threat investigations run by MI5 has “shot up by 48%”, he said.

Britain should “expect to see continued acts of aggression here at home” from Russia, with its military intelligence agency the GRU on “sustained missions to generate mayhem” on the country’s streets, he said, adding: “We’ve seen arson, sabotage and more.”

Russia’s embassy in London disputed Mr McCallum’s statement, describing it as “unsubstantiated” and claiming Russia itself was “the real target of proxy and mercenary warfare as well as hostile intelligence operations” conducted by the UK and its Nato allies.

Turning to Iran, Mr McCallum said there has been an “unprecedented pace and scale” of plots since 2022, posing “potentially lethal threats” to British citizens and UK residents.

As events unfold in the Middle East, MI5 is giving its “fullest attention” to the risk of a rise in Iranian-state backed aggression in the UK, he added.

Overall, MI5 and the police have disrupted 43 late-stage attack plots since March 2017, saving “numerous lives”, he said, adding: “Some of those plotters were trying to get hold of firearms and explosives, in the final days of planning mass murder.”

Mr McCallum said MI5 was “powerfully alive to the risk that events in the Middle East directly trigger terrorist action in the UK”, and while the “ripples from conflict in that region will not necessarily arrive at our shores in a straightforward fashion, they will be filtered through the lens of online media and mixed with existing views and grievances in unpredictable ways”.

Although police had responded to “rising public order, hate crime and community safety challenges, we haven’t, yet, seen this translate at scale into terrorist violence”, he said, adding: “The overarching UK terrorist threat level remains at substantial – an attack is likely – and is kept under constant review.”

Mr McCallum said the terrorist trend that concerned him most was the “worsening threat from al Qaida and in particular from Islamic State”, adding: “Islamic State is not the force it was a decade ago. But after a few years of being pinned well back, they’ve resumed efforts to export terrorism.”

Setting out his concerns at the rise in the number of children MI5 are encountering, he said 13% of people being investigated by the security agency for involvement in UK terrorism are under 18.

The internet is driving the trend behind the threefold increase seen in the last three years, he said, adding that extreme right-wing terrorism “in particular skews heavily towards young people, driven by propaganda that shows a canny understanding of online culture”.

Alluding to the summer riots, he also spoke of the “insidious effect of internet hatred and disinformation has played into threats to election candidates, intimidation of communities, and the public disorder that followed the sickening attack in Southport”.

Sir Keir Starmer acknowledged the “sober findings” outlined in Mr McCallum’s speech, but the public should be “reassured that our security services are world class and will do everything necessary to keep us safe,” a Number 10 spokeswoman said, adding: “The Prime Minister would like to take this opportunity to thank the security services for the work that they do day in and day out to keep us all safe.

“These rare public interventions are a reminder of the work they do without public recognition.

“What was very clear from the director general is we remain alert and vigilant to all risks, and in turn we will always ensure that our police and security services have the tools that they need to manage the evolving threats that we face.”