LONDON, U.K.: The head of Britain's MI5 intelligence service has warned that security agencies must not focus too heavily on threats from the past.

He said they must also consider the possibility that a major new attack, similar to the September 11, 2001 attacks, could already be being planned somewhere they are not looking.

MI5 Director General Ken McCallum made the warning in an opinion article published on September 11 to mark 25 years since al Qaeda militants hijacked passenger planes and crashed them into the World Trade Center in New York and the Pentagon.

McCallum said intelligence and security agencies needed to learn from previous attacks while remaining ready to address new and evolving threats.

"We know that the next strategic shock will, by definition, not resemble the last one. That is perhaps the most important lesson of all," McCallum wrote in the Independent newspaper.

He said security agencies could make a serious mistake by focusing too much on fighting enemies and using methods they had used in the past. He warned that a future major attack could already be developing in an area that authorities had not yet identified.

McCallum said al Qaeda had been weakened but remained a threat. He also warned that the desire among al Qaeda and other militant groups to carry out attacks causing large numbers of deaths remained a serious concern.

He pointed to a recently stopped Islamist plot as an example. The plot was aimed at killing hundreds of people in gun attacks against the Jewish community in Manchester, northern England.

However, McCallum said the threat facing Britain had changed since 2001. MI5 was now also dealing with increasing aggression backed by foreign states. This included sabotage carried out by criminal groups and cyberattacks against critical infrastructure, such as power stations.

He said another major difference was that many essential systems and services were now controlled or operated by private companies rather than directly by the government.

Because of this, governments, businesses and universities would need to work together to identify weaknesses and respond quickly during a crisis.

McCallum warned that authorities should not wait for another major attack before deciding who was responsible for addressing a weakness in the system or who had the authority to act.

He also said the victims of the September 11 attacks should be remembered through more than ceremonies and memorials. He argued that governments and security agencies needed to prepare for future threats, strengthen society's ability to withstand attacks and be willing to change their approach when new dangers emerged.

His comments came as Britain and other countries marked 25 years since the September 11 attacks, which killed nearly 3,000 people and led to major changes in global security and counterterrorism efforts.