The Metropolitan police force has taken the unprecedented step of writing to parents of school-age children, urging them to look out for signs of radicalisation because it fears the six-week summer holiday could lead to a rise in extremism.

Det Supt Jane Corrigan, of the Met’s counter-terrorism command and lead officer in the anti-terrorist Prevent programme, sent a letter to primary and secondary schools in London – the first time such a step has been taken – to distribute to parents last week. In it she expresses concern that children would be spending more time online during the summer holidays, and that this would create the risk they could come into contact with those attempting to radicalise young people.

She advised parents to use the ACT Early website to identify signs of radicalisation, such as becoming obsessive or expressing extreme views, and to contact Prevent for support.

The government’s independent reviewer of terrorism legislation welcomed the development, saying the police had concluded that tackling the radicalisation of youngsters required society’s efforts as a whole.

Jonathan Hall QC said: “What’s so striking is that counter-terrorism usually operates behind the scenes – they have a minimal public presence – and that’s why this letter seems really important. They’re coming out and saying: ‘We can’t do it on our own’.”

Prevent is intended to divert people away from extremist ideologies. Corrigan manages London’s vulnerability support hub, which works with psychiatrists, psychologists and nursing staff. “Our job is really about making sure that we catch people and support them before it’s too late,” she said. “The purpose of my letter was to make sure that we appeal to parents, because they are usually the ones that will identify that deterioration, that vulnerability.”

Corrigan said about 30% of Prevent referrals came from schools, so the letter was important to ensure children did not slip through the net when not being seen by teachers.

She said that police often did not need to intervene because families were already receiving support from children’s mental health services, education support workers or social services. “If you think they’re vulnerable to being radicalised and need support, then call the advice line and we will ensure they get the support they need,” she added. “That’s our job, and sometimes that means difficult conversations with statutory partners.”

Corrigan said that the nature of terrorism threats had evolved from groups with clear ideological motives to individuals often described as “lone actors” with “mixed, unstable or unclear ideologies”, which accounted for more than half of the referrals to Prevent across the country.

“We’ve also seen that those ideologies are diversifying, and they’re becoming less fixed,” Corrigan said. “Subjects are often picking and choosing extremist contents from a range of sources. So people are kind of flip-flopping between ideologies and beliefs, and in most but not all cases, they’re inspired to conduct low sophistication attacks.”

Vulnerable and marginalised people were often targeted by extremists, Corrigan added, noting that an Islamic State promotional video included a sign language interpreter. “The reason they’ve got somebody signing is because they’re trying to reach the deaf community. Who is thinking about approaching the deaf community in terms of them being radicalised? It’s not something that automatically springs to mind.”

London continued to see more referrals for Islamist threats than extreme rightwing ones, a situation “at odds with the rest of the country”, she said.

In May, a leaked version of a report by William Shawcross, Prevent’s independent reviewer, said that the programme was “carrying the weight for mental health services” because of the lack of resources, and that people had been referred simply to access other types of support.

The issue of youngsters being drawn into extremism has posed an increasing challenge for police and the intelligence agencies with a recent speech by Hall warning that teenagers suspected of sharing and promoting terrorist material online should be spared prosecution if they were just “keyboard warriors”.

Data shows that of the 20 under-18s arrested in 2021, only five were charged and one convicted, suggesting that the police were aware of the difference in threat between youngsters posturing online and a genuine terrorist.