Ten out of the 12 under-18s who were arrested for terrorism last year were linked to extreme right-wing beliefs, police figures show.

The proportion is high compared to data for all age groups, which shows that 20% of terrorism arrests in the year to June 30 2020 were linked to the ideology.

Head of UK counter-terrorism policing Neil Basu said that while Islamist terrorism remains the greatest threat and makes up 80% of his workload, right wing extremism is the fastest growing threat and has grown from 6% to 10% of his work in the past few years.

He told the PA news agency: “There has definitely been a growth in nationalistic material online, white supremacist literature, things that are extremely disturbing in the extreme right wing space.

“Many of which do hit criminal thresholds, some of which is designed to entice people into closed groups where criminality and Nazi ideology is being discussed. It does seem to be having an effect on younger and younger children.

“In our case work in the extreme right wing space the subjects of interest to us do appear to be younger.”

He added: “My warning has always been that the Islamist threat is still the greatest threat we face, but the one that’s growing fastest, from a relatively low base, is the right-wing terrorism threat.”

A new website – actearly.uk – is being launched to help concerned parents get advice on how to deal with teenagers who may be showing signs of radicalisation.

Terrorist groomers from both Islamist and extreme right wing ideologies deliberately target vulnerable people including children.

Mr Basu said: “We have seen young and vulnerable people both with complex psychological needs, but also just young and curious, being drawn through mainstream websites into encrypted chat rooms and closed groups.

“That’s when the really serious radicalisation and extreme ideologies start. It is entirely possible for those people to be groomed and radicalised in a very, very short period of time.”

Figures show that a total of 12 under-18s were arrested for terror offences in 2019, of whom 10 were linked to extreme right wing beliefs.

In the 18 months to June 30, 2020, a total of 17 under 18s were arrested on suspicion of terrorism, some as young as 14. Police said most will have been radicalised entirely online.

During that period police say 1,500 children under the age of 15 were helped by the Prevent programme, a government scheme to divert people away from terrorism.

Concerns have been raised that lockdown conditions have given extremists more opportunity to radicalise children.

Mr Basu said: “I’m worried that the radicalisation of some of the most vulnerable people in our society, namely our children, is happening by online groomers and terrorists both from the Islamist and extreme right wing ideologies.

“It’s that online radicalisation, the explosion of online and technological devices in people’s hands 24/7, on top of the pandemic, which has effectively led to a lot more time people are spending on those devices, locked in their rooms away from their protective influences, while they’ve been out of school or out of colleges.

“It’s all given more time for their radicalisation to take place. The people who are going to stop it are the people who love them the most, who are the friends and family that see that change in behaviour.”

He said that if people don’t want to speak to the police, the website has details of third parties including well known charities that have trained advisors who can help.

“It might be very difficult to understand what looks like normal teenage behaviour and what looks like the early signs of damage or grooming that is being done to your child online.

“I have some sympathy with parents who don’t understand or know how to monitor their child’s activity online and don’t know how to step in.

“The best technique has got to be ‘prevention is better than cure’. Let’s reach people before they get to the stage where they cross the Rubicon into terrorism.

“We want to stop people becoming criminalised, we want to stop them becoming terrorists. The way to do that is to act early.”

Read more: French leader decries Islamist terror attack against teacher

Right-wing extremism is the fastest growing terrorist threat in the UK – with supporters seeking to mimic jihadist attacks, police have said.

The UK head of counter-terrorism Neil Basu said seven of the 22 plots foiled since March 2017 have been linked to far-right ideologies.

About 10% of around 800 live terror investigations were linked to right-wing terrorism, while the Government’s controversial Prevent programme has seen referrals nearly doubling since 2015/16 to 18%.

“It’s rising from a low base, but it’s probably the fastest growing bit of my casebook at the moment,” the Metropolitan Police Assistant Commissioner said at a briefing on Thursday

“I see it as my job, and the security service as my major partner, to stop that.”

Mr Basu said counter-terror police and MI5 have disrupted right-wing plots “designed to kill people”.

Attack methods used and planned have mimicked those seen in jihadist plots, including knife attacks and seeking to create improvised explosive devices, he told reporters.

Mr Basu said some of the Islamic State guidance materials were also being used by right-wing terrorists, although they tend to target specific demographics or individuals rather than launching indiscriminate attacks in a public place.

And he said the biggest threat still comes from Islamist and jihadist terrorists.

“Despite the increases, right-wing terrorism remains a relatively small percentage of our overall demand, but when nearly a third of the plots foiled by police and security services since 2017 relate to right-wing ideology, it lays bare why we are taking this so seriously,” he said.

“As a proportion of our overall threat it’s definitely increasing, whereas the Islamist threat is staying the same, albeit at a very high level.”

Mr Basu said the National Action group has been “decimated” since it was banned in December 2016 after members publicly praised Thomas Mair for the terrorist murder of Labour MP Jo Cox.

Right-wing extremists now operate within small groups across international borders online, with lone actors, who can be radicalised within weeks or days, posing the greatest threat, he added.

The counter-terror chief said young people and those with mental health issues are particularly vulnerable to becoming radicalised, with children as young as 14 linked to right-wing terrorism.

University students and members of the military have also been targeted.

“We are bringing the full might of the UK counter-terrorism machine to bear against those extremists of any ideology who wish to do us harm or incite violence,” Mr Basu said.

“And that is evident not only in the number of plots we have foiled, but also the number of convictions we have achieved and continue to chase through the courts.”

Last week, white supremacist Vincent Fuller, 50, was jailed for 18 years and nine months for stabbing 19-year-old Dimitar Mihaylov in what a judge called a “terrorist act”.

Three other right-wing terrorists, including a “16-year-old Nazi”, from Bradford, who was convicted of making a bomb in a bid to “kill many people” are due to be sentenced in Leeds and London on Friday.

But Mr Basu said police “can’t arrest ourselves out of this problem” and called on the public to come forward if they fear a friend or family member is becoming radicalised.

“I have been called ridiculously idealistic, but I believe more than ever that evil triumphs when good people do nothing,” he said.

“In the UK the Prevent programme is full of people who get up every day to do something to protect those who need our help, and I believe that is worth defending and fighting for.”

Mr Basu said both hate crime and right-wing terrorism had increased since the EU referendum, but pointed out that spikes were also seen after incidents such as Islamist attacks.

“This is not just about Brexit or the referendum. It’s an international problem,” he added.