The chief of Britain's domestic intelligence service on Tuesday blamed extreme right-wing ideologies for a "staggering" rise in the number of children being investigated for terrorism.

MI5 boss Ken McCallum also said that Russian intelligence was seeking to cause "mayhem" in Britain because of its support for Ukraine and that his agency had investigated growing numbers of Iran-backed plots.

Under-18s represent 13 percent of people being investigated by the spy agency for possible involvement in terror activities, McCallum said.

He told reporters at MI5's Counter Terrorism Operations Centre in London that the number marked "a threefold increase in the last three years".

McCallum said the internet was the "biggest factor" driving the rise, describing how easily youngsters can access "inspirational and instructional material" from their bedrooms.

He said the intelligence service was seeing "far too many cases where very young people are being drawn into poisonous online extremism" and singled out "canny" internet memes.

"Extreme right-wing terrorism in particular skews heavily towards young people, driven by propaganda that shows a canny understanding of online culture," he added.

"It's not really a consistent single ideology on the extreme right-wing side and that is what has skewed the numbers most heavily."

The UK terrorist threat level remains at "substantial" – the third highest on five – meaning an attack is likely.

MI5 and the police have disrupted more than 40 late-stage attack plots since March 2017, saving "numerous lives", McCallum said.

According to McCallum, Russia's intelligence service had tried to cause "mayhem" in the UK because of Britain's support for Ukraine.

Meanwhile, MI5 has responded to 20 Iran-backed plots since January 2022 that presented potentially lethal threats to British citizens and UK residents, he added. (AFP)