Counter-terrorism police fear that uncertainty over the future of Syria could fuel jihadi attacks in the UK.
Border officials are on high alert for potential British ISIS supporters freed from jail who may try to return to Britain.
Vicki Evans, national counter-terrorism coordinator, said the current terror threat was “smouldering” and had never been more complex with the danger posed by Islamic extremists, state-sponsored plots and planned attacks from individuals with no obvious ideology.
“Events in Syria are certainly something that are a focus and something that all of us need to think about with our partners,” she said.
“It’s that stark reminder that we need to focus on old enemies of peace and security as well as the new.
“History tells us that, unfortunately, any instability creates space for extremism, for violence and acts of terror.”
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Deputy Assistant Commissioner Evans said although the British government had engaged diplomatically with the de facto new Syrian leader Ahmed al-Sharaa, his victorious rebel group Hayat Tahrir al Sham (HTS) remains a banned terror group under UK law and anyone demonstrating support for it could be charged with a terror crime.
She said no such suspects had been arrested, but would not confirm or deny if any were under investigation.
DAC Evans also revealed that counter-terrorism police were increasingly finding images of extreme violence, pornography, misogyny and gore that sometimes fuelled terror plans in the online viewing history of suspects as young as 10.
She said: “It’s a pick-and-mix of horror. These sorts of grotesque fascinations with violence and harmful views that we’re seeing are increasingly common.
“We most definitely need to think differently about how we stop that conveyor belt of young people who are seeing and being exposed to this type of material and, unfortunately, sometimes then going on to commit horrific acts.”
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She added that police had disrupted 43 plots – three in the past year – that were close to being carried out, all with potential mass casualties, since 2017, a year of multiple UK terror attacks.
Plots backed by hostile states had quadrupled in the past few years, with 20 detected since 2022 that were sponsored by Iran, she said.
She urged the public to be vigilant and appealed to businesses to beware of their vulnerability to hostile state attacks.