Sir Ken McCallum, the Director General of MI5, has warned that Britain now faces a greater security threat than it did at the time of the September 11 attacks, marking the 25th anniversary of the atrocity with a stark "call to arms". He said the UK is confronting a combined threat from hostile states including Russia, China and Iran alongside enduring Islamic extremist terrorism, both increasingly amplified by artificial intelligence, meaning Britain must contend with what he called "the worst of both worlds".

Writing in The Independent, McCallum contrasted today's landscape with 2001, recalling his own experience working in MI5's small counter-terrorism team when the Twin Towers were struck, and how the fight against al-Qaeda forced a radical overhaul in intelligence-sharing. While he said that international effort ultimately succeeded in reducing al-Qaeda's capabilities, he stressed MI5 cannot rely on old tactics, citing new dangers such as state-backed sabotage, covert interference in democracy and cyberattacks on infrastructure, much of which now lies in private sector hands, requiring closer cooperation across government, industry and academia.