Twenty-five years after the September 11 attacks transformed global security policy, Britain’s intelligence chief has issued a warning that the next major terrorist or strategic shock could come from a threat that authorities are not currently watching closely enough.

MI5 Director General Ken McCallum has urged intelligence agencies and governments to remain alert to emerging risks, arguing that the biggest danger may not necessarily resemble the attacks that have shaped security thinking over the past two decades.

In an article marking the 25th anniversary of the 9/11 attacks, McCallum stressed the importance of avoiding a narrow focus on threats from the past. His warning comes at a time when Western intelligence agencies are confronting a much more complicated security environment involving terrorism, state-backed activity, cyber operations, sabotage and rapidly developing technology.

The danger of looking only at familiar threats

The September 11 attacks demonstrated how a relatively small terrorist organisation could exploit gaps in intelligence and security to produce consequences far beyond the scale that authorities had anticipated.

McCallum's central message is that intelligence organisations must continually reassess where the next major threat could originate. A future crisis may not necessarily follow the pattern of previous terrorist attacks, making adaptability and early detection increasingly important.

Although al-Qaeda's capabilities have changed significantly since 2001, extremist organisations continue to seek opportunities to conduct mass-casualty attacks. British authorities have also dealt with more recent terrorism-related threats, including a plot targeting Manchester's Jewish community that was disrupted by security agencies.

Threat landscape is becoming more complicated

The warning extends beyond traditional terrorism.

British intelligence officials are increasingly concerned about a wider range of hostile activities, including cyberattacks, sabotage and operations linked to foreign states. The UK's security environment has consequently become more difficult to predict, with threats potentially emerging simultaneously across physical and digital infrastructure.

Britain's national terrorism threat level was raised to SEVERE in April 2026, meaning authorities assessed an attack as highly likely. The UK government said the increase reflected a broader Islamist and extreme-right-wing terrorist threat, including activity involving individuals and small groups operating inside the country.

Separately, British intelligence has warned about hostile state activity. GCHQ Director Anne Keast-Butler previously said Russia was targeting critical infrastructure, democratic processes, supply chains and public trust, highlighting how modern security threats increasingly extend beyond conventional terrorism.

Why the 9/11 anniversary matters

The warning carries particular significance because September 11, 2026 marks 25 years since the attacks on the United States that killed nearly 3,000 people and fundamentally changed international counter-terrorism policy.

The attacks also became a major lesson for intelligence agencies about the dangers of fragmented information and failing to recognise warning signs before a crisis.

A quarter-century later, the challenge is different. Security agencies must now monitor terrorist networks while also tracking cyber threats, foreign interference, sabotage, technological developments and other forms of unconventional warfare.

McCallum's message is therefore less about predicting a specific future attack and more about warning against complacency.

Intelligence agencies face a new kind of uncertainty

One of the biggest challenges for governments is determining which emerging threats deserve attention before they become obvious.

An attack or crisis that appears unlikely today could become significant tomorrow because of changes in technology, geopolitical tensions or the ability of hostile actors to exploit vulnerabilities.

Recent incidents involving suspected foreign sabotage operations have demonstrated how national security risks can increasingly exist below the threshold of conventional warfare. NATO allies, for example, have recently reported disrupting a suspected Russian operation involving undersea infrastructure near Svalbard.

That changing landscape means intelligence agencies are being pushed to think beyond traditional terrorism scenarios.

From 9/11 lessons to future preparedness

McCallum's warning ultimately points to a broader principle: intelligence failures can occur when institutions become too comfortable with established assumptions.

The next major security crisis may not look like 9/11. It could involve a combination of physical attacks, cyber disruption, critical infrastructure failures, disinformation or other methods designed to create widespread instability.

For governments, businesses and security agencies, the challenge is therefore not simply to identify today's threats but to prepare for threats that have not yet become obvious.