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, in an op-ed published in the Independent on Friday.
He said 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.
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 Qaida 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 Qaida's capabilities are a "fraction" of what they were in 2001.
"The horrors of 9/11 will never leave me," he wrote.
"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."
