Britain’s domestic intelligence chief has warned that security agencies must prepare for threats that may look very different from the attacks of the past. MI5 Director General Ken McCallum issued the warning on Friday as Britain and other countries marked the 25th anniversary of the September 11 attacks.

McCallum said intelligence services should learn from previous attacks but avoid becoming too focused on yesterday’s threats. He warned that a future major security crisis could already be developing in an area that authorities are not currently watching closely.

According to the MI5 chief, extremist organisations remain capable of attempting mass-casualty attacks even though al-Qaeda has been significantly weakened. He also pointed to the growing threat from state-backed actors, including cyberattacks, sabotage and the use of criminal groups as proxies.

The warning comes amid a broader increase in security concerns across Europe. NATO countries have recently increased surveillance of critical infrastructure after an alleged Russian operation involving undersea communications cables near Svalbard was disrupted. Britain was among the countries involved in responding to that activity.

British authorities are also dealing with suspected foreign-intelligence activity. London police recently arrested two people under the National Security Act in an investigation linked to Iran. Both suspects were released on bail while the investigation continues.

McCallum stressed that protecting modern society requires cooperation between governments, businesses and academic institutions because many critical systems are now operated outside the state sector.

The latest warning highlights how Britain’s security priorities are expanding beyond conventional terrorism to include cyber threats, sabotage, hostile states and attacks on critical infrastructure.