Teenagers who share extremist content online are risking a prison sentence without leaving their bedrooms, the head of Counter Terrorism Policing has said.

Matt Jukes also told the BBC officers were seeing more online hatred spill over into “real threats of violence on the streets”.

Under-18s accounted for 14% of all terrorism-related arrests in the UK in the year to March 2023, down from a record 15% in the previous 12 months, Home Office figures show.

It marks a significant rise on 2019/20, when it was 5%.

Some 24 people under the age of 18 were arrested for terror-related activity in the year to March 2023, down from 29 in the previous 12 months.

The overall number of terror arrests in the UK has been on a downward path in recent years, dropping from a record 447 in 2017/18 to 169 in 2022/23.

Mr Jukes described the rise in the number of younger terror suspects since 2019 as a “really worrying trend”.

He told BBC Radio 4’s Today programme: “The really remarkable thing is now that literally, you can find yourself with a prison sentence not having left that bedroom, because of encouraging, inciting, sharing information, downloading bomb instructions and encouraging other people to take part in acts of violence.

“And sadly we’re seeing more of this translate into real threats of violence on the streets.”

He added that counter terrorism officers have seen young people downloading instructions for 3D printed firearms, buying chemicals online and the reconnaissance of potential targets.

Mr Jukes said: “We know that the driving threat here is online but we also can see in our casework that for younger people than ever before, that’s translating into the prospect of them actually carrying out acts of violence.”

He said the trend was a “real concern” and a “contagion”, citing the case of Daniel Harris, 19, from Derbyshire, who was convicted over videos he had posted online, which were then shared by a man who carried out an attack in Buffalo, New York.

Payton Gendron posted a video in May last year showing him carrying out an attack at Tops supermarket in the US city, leaving 10 dead.

Mr Jukes told the BBC: “But it’s one in five of the arrests we make now involve someone under 18, and more of them involving under 25s, so this is a new and emerging threat, which should concern us all and certainly should concern us in respect to the online lives of our communities.”