MI5 chief warns next major threat may come from where the UK is not looking

The UK’s next major security shock could emerge from a threat intelligence agencies are not currently watching, MI5 Director General Sir Ken McCallum has warned on the 25th anniversary of the September 11 attacks.

Writing in a commentary reported by Reuters, McCallum said one of the main lessons of 9/11 was the danger of preparing for past threats while adversaries change their methods.

“We know that the next strategic shock will, by definition, not resemble the last one. That is perhaps the most important lesson of all,” he wrote. “We must of course always learn from the last attack – but must beware of preparing brilliantly to fight yesterday’s enemy, using yesterday’s methods, against yesterday’s target.”

He added: “The uncomfortable truth is that the next strategic shock may already be taking shape somewhere we are not yet looking.”

McCallum was not warning of intelligence about a specific 9/11-scale plot. His comments reflect the broader range of threats now facing the UK, including terrorism, hostile states, cyberattacks, sabotage and emerging technologies.

UK terrorism threat remains severe

The warning comes as the UK’s national terrorism threat level remains SEVERE, meaning an attack is considered highly likely. The Joint Terrorism Analysis Centre raised the threat level from SUBSTANTIAL in April 2026.

Terrorism has also changed since 9/11. MI5 increasingly deals with individuals and small groups rather than large, organised networks.

In an October 2025 threat update, McCallum said MI5 and police had disrupted 19 late-stage attack plots since the start of 2020.

He also warned that al Qaeda and Islamic State were becoming more ambitious as instability overseas gave them opportunities to rebuild.

Hostile states pose a growing threat

MI5 is simultaneously dealing with a sharp rise in hostile-state activity. McCallum said in 2025 that the number of people investigated for suspected involvement in state-threat activity had risen 35% in a year.

The threat extends to cyberattacks, surveillance, sabotage, arson and potential violence. 

Russia and Iran have been particular concerns. Hostile governments use criminals and other proxies to conduct operations, making it harder to link attacks directly to the state actor.

Cyberattacks expose new vulnerabilities

The UK’s dependence on interconnected digital infrastructure has created risks that barely existed when the 9/11 attacks occurred.

Transport, communications, energy and other critical systems increasingly depend on cooperation between government agencies and private companies.

“We cannot wait for the next shock to discover who owns the vulnerability, who has the authority to act, or who was supposed to pick up the phone,” McCallum warned.

MI5’s National Protective Security Authority works with organisations responsible for critical infrastructure and sensitive assets, while the National Cyber Security Centre focuses on cyber threats.

AI changes the security landscape

McCallum has warned that technology can strengthen almost every category of national-security threat. Terrorists can use Artificial Intelligence for propaganda, research and reconnaissance. Hostile states can use it for cyberattacks and influence operations.

At the same time, AI can be useful in defence. MI5 is using AI to analyse intelligence and identify potential threats.

For McCallum, the lesson from September 11 is not just to prepare for another similar terrorist attack. It is to anticipate threats that may look completely different.

“We owe the victims of 9/11 – and those who will otherwise become the victims of tomorrow’s attack – something more than remembrance,” he wrote. “We owe them preparation, resilience and the courage to adapt before the next shock arrives.”