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'The horrors of 9/11 will never leave me': MI5 chief warns Britain now faces greater danger than 25 years ago

Sir Ken McCallum, director general of the UK’s security service, has underlined the need to adapt to “markedly different” threats than those faced on September 11 2001

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Director General of MI5 Sir Ken McCallum, delivers the annual Director General's Speech at Thames House, the headquarters of the UK's Security Service on October 16, 2025
Director General of MI5 Sir Ken McCallum, delivers the annual Director General's Speech at Thames House, the headquarters of the UK's Security Service on October 16, 2025. Picture: Jonathan Brady - WPA Pool/Getty Images

By Rebecca Henrys

Intelligence services must not dwell too much on past threats but accept that a new strategic shock along the lines of the 9/11 attacks could already be in preparation where they are not looking, the head of MI5 said.

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Sir Ken McCallum, director general of the UK’s security service, has underlined the need to adapt to “markedly different” threats than those faced on September 11 2001.

He wrote in an op-ed published in the Independent that security needed to be treated as a “shared national endeavour” as the victims of 9/11 should be owed “preparation, resilience and the courage to adapt”.

“We must resist the temptation to admire the defences we built yesterday and instead build the ones we will need tomorrow,” he wrote.

Read more: Burnham warns terror 'still knocks at the door of democracy' on 9/11 anniversary as Blair recalls day that 'changed the world'

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A fiery blast rocks the World Trade Center after being hit by two planes September 11, 2001 in New York City
A fiery blast rocks the World Trade Center after being hit by two planes September 11, 2001 in New York City. Picture: Spencer Platt/Getty Images

Mr McCallum, who was appointed as director general in 2020 and has spent more than 25 years in the service, said he was “proud” of the work MI5 did in disrupting al Qaeda in the wake of September 11.

He noted the “relentless adaptation” central to that response, which remains pivotal for “still more complex” challenges in the 2020s.

The MI5 boss did not go into the often controversial war in Afghanistan that followed, led by US and UK forces, other than to say that foreign policy is “harder than many commentators allow”.

He added that al Qaeda’s capabilities are a “fraction” of what they were in 2001.

Looking to modern threats, Mr McCallum spoke about the recent attacks on synagogues in the UK and how many of the “assets, systems and services under threat” are now owned by the private sector, rather than by nations.

He wrote: “In the aftermath of the Cold War, CIA director James Woolsey spoke of the West having 'slain a large dragon' but facing a 'bewildering variety of poisonous snakes'.

“With a nod to David Kilcullen, we now face the worst of both worlds: dragons are back, but snakes persist.

He added: “The horrors of 9/11 will never leave me. What sits with me every day, 25 years on, is MI5’s perpetual responsibility to keep adapting, at even greater pace, as we seek to deepen the UK’s resilience – now to both dragons and snakes.”