Britain must remain prepared for a major future security crisis that could emerge from an unexpected direction, MI5 Director General Ken McCallum has warned.

Writing on the 25th anniversary of the September 11 attacks, McCallum said intelligence agencies must avoid becoming too focused on threats from the past and instead prepare for new and unpredictable forms of danger.

He warned that extremist organisations continue to seek opportunities to cause mass-casualty attacks, while Britain also faces growing threats from state-backed cyberattacks, sabotage and other hostile activity.

McCallum's warning comes after British authorities recently disrupted an alleged plot involving suspected Russian intelligence activity and a UK drone manufacturing facility, underlining the increasingly diverse nature of national-security threats.

The MI5 chief argued that preparing for future crises cannot be the responsibility of intelligence agencies alone. Government, businesses and universities will also need stronger resilience and coordination as the security environment becomes more complex.

The warning has renewed attention on Britain's ability to identify threats before they develop into major national emergencies.