A 13-year-old was investigated by MI5 over extreme right-wing beliefs earlier this year, the head of the security service said.
Sir Ken McCallum told journalists the recent increase in the terror threat to the UK was “driven in significant part” by right-wing extremists.
In a speech at Thames House on Tuesday, he said: “Over the last year, the most marked trend has been an increase in cases where extremist versions of anti-immigration and anti-Muslim grievance are tipping into terrorist intent; we’ve dealt, for example, with would-be terrorists aspiring to attack mosques and venues connected to immigration and asylum.
“Extreme right-wing terrorism also continues to skew (the) young, driven by the internet.
“A few months ago, MI5 was investigating a radicalised extreme right-wing 13-year-old here in the UK.
“Believe me, we wish we didn’t have to. But that investigation generated insight that enabled one of our international partners to prevent a lethal attack on their soil.”
He also mentioned the case of a 16-year-old from Inverclyde who was jailed for 10 years for plotting to attack an Islamic centre.
Official figures show that in 2025, 39 people aged 17 and under were arrested in connection with counter terrorism investigations, and 40 the previous year.