Britain is confronting a security threat more severe than during the period immediately following the 9/11 terror attacks, according to the head of MI5, Sir Ken McCallum. In a statement marking the 25th anniversary of 9/11, McCallum described the current landscape as facing "unprecedented danger" from both hostile nation-states and persistent extremist groups.

McCallum, leading the UK's Security Service, stated that the threat has intensified and diversified since 2001. He noted that while the ambition for mass-casualty terrorism endures, it now exists alongside strategic national security threats posed by countries such as Russia, China, and Iran. These state actors, he warned, are exploiting technological advancements, including artificial intelligence, to conduct activities ranging from sabotage and covert interference in democracy to cyberattacks on critical infrastructure.

"We now face the worst of both worlds: dragons are back, but snakes persist," McCallum stated, referencing a metaphor previously used by former CIA director James Woolsey. He explained that the MI5 of 2026 contends with a "markedly different" threat landscape compared to 2001.

Reflecting on the 9/11 attacks, McCallum recounted being on an MI5 counter-terrorism team when news of the planes hitting the World Trade Center broke. The subsequent effort to dismantle al-Qaeda, he noted, led to a significant transformation within MI5, fostering intelligence sharing with domestic and international partners and a surge in recruitment. While al-Qaeda's capabilities have been reduced, McCallum emphasized that MI5 cannot rely on outdated tactics.

He highlighted that national security in 2026 depends on collaboration beyond government agencies, involving the private sector, academia, and the public. "In 2026, unlike 2001, many of the assets, systems and services under threat sit in private sector hands," he wrote. "This means many of the most critical decisions will be taken not in Cobra but in boardrooms." McCallum issued a "call to arms," urging that national security be treated as a "shared national endeavor."

A Downing Street spokesperson acknowledged the growing scale and complexity of threats from foreign powers and their proxies, which endanger lives and democratic values. The spokesperson confirmed the fast-tracking of the National Security Act 2026, providing new powers to designate state-backed organizations and proxy groups that threaten national security.