LONDON –

Russia's GRU military intelligence service is seeking to cause "mayhem" across Britain and Europe, the U.K.'s domestic spy chief said on Tuesday, while underlining a growing threat from resurgent al-Qaeda and the Islamic State group as his greatest terrorism concern.

In a wide-ranging speech outlining the current threat picture, Security Service (MI5) Director General Ken McCallum also accused Iran of being behind "plot after plot" on British soil.

McCallum said state threat investigations were up 48% in the last year as Russia and Iran turned to criminals, drug traffickers and proxies to carry out their "dirty work."

"It will be clear to you that MI5 has one hell of a job on its hands," he said. Since March 2017, he said MI5 and the British police had disrupted 43 late-stage plots, some of which were in the final days of planning for mass murder.

The terrorist trend that the spy chief said concerned him most was a worsening threat from al-Qaeda seeking to capitalize on the conflict in the Middle East, and especially the Islamic State group, which had resumed efforts to export terrorism, such as the March attack on a Moscow concert hall.

Events in the Middle East had not yet translated at scale into militant violence at home, he said. "We are powerfully alive to the risk that events in the Middle East directly trigger terrorist action in the U.K.," he said, citing an incident where a Moroccan man stabbed to death a passerby in northern England in what he said was revenge for Israeli actions in the Gaza Strip.

Of those people being investigated for involvement in terrorism, 13% were under the age of 18, a threefold increase in the last three years, he said.

Much of McCallum's speech was taken up with the state threats posed by Russia and Iran. With the expulsion of more than 750 Russian diplomats from Europe since Russia invaded Ukraine and the ejection of the last Russian military intelligence officer from Britain this year, he said it was "eye-catching" how Russian state actors were turning to proxies to do their work.

"The GRU in particular is on a sustained mission to generate mayhem on British and European streets: we've seen arson, sabotage and more," he said, declining to give further details.

The Russian Embassy in London said it emphatically rejected the "unsubstantiated allegations," describing McCallum as using scare tactics to sustain public backing for Britain's support of Ukraine.

Since January 2022, McCallum said his service and the police had responded to 20 Iranian-backed plots, which potentially posed lethal threats to U.K. citizens and residents.

"We've seen plot after plot here in the U.K., at an unprecedented pace and scale," he said. There was no immediate response from the Iranian foreign ministry.

Last December, an Austrian man was found guilty of carrying out "hostile reconnaissance" against London-based television station Iran International, which is critical of Iran's government.

Since then, Britain has imposed sanctions on Iranian officials it accused of being involved in plots to assassinate two Iran International presenters, while in March, a counterterrorism investigation was launched into the stabbing of British-based Iranian journalist Pouria Zeraati.

McCallum warned anyone who might take money from Russia or Iran to carry out illegal actions that it would be "a choice you'll regret."