Sir Ken McCallum, the head of MI5, has used an exclusive op-ed for The Independent marking the 25th anniversary of 9/11 to warn that Britain now faces a more dangerous and complex threat landscape than it did in 2001. He argues that alongside the enduring menace of Islamist terrorism, the UK must now also contend with aggressive state actors, principally Russia and China, describing this combined danger as the "worst of both worlds". His intervention is unusually direct for an MI5 chief and amounts to a public "call to arms", urging that national security be treated as a shared responsibility across government, business, academia and the public rather than the preserve of a few agencies.

McCallum notes the UK's threat level remains at "severe", meaning an attack is highly likely, even as al-Qaeda's capabilities have been vastly reduced since the multinational campaign that followed the 2001 attacks, which killed 2,977 people. He warns against complacency, saying Britain must "build the defences we will need tomorrow" rather than admire past successes, and highlights growing state-backed threats such as sabotage outsourced to criminals, interference in democracy, and cyber attacks on infrastructure like power stations. His comments echo warnings from other figures, including the late former MI6 chief Sir Alex Younger and ex-White House adviser Fiona Hill, who have similarly called for a "whole nation" approach to resilience against Russian and Chinese threats.