Right-wing extremism is the fastest growing terrorist threat in the UK, while Islamist terrorists still pose the biggest threat.
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 - nearly a third.
This "lays bare" why security services are taking the threat so seriously, he added.
About 10 per cent of more than 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 per cent.
- ITV News Correspondent Rebecca Barry reports live after hearing from police
The Metropolitan Police Assistant Commissioner said at a briefing on Thursday: “It’s small but it’s my fastest-growing problem".
But Mr Basu 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.”
Since the group National Action was banned three years ago, they are no longer seeing physical groups of people gathering.
Instead this is about individuals - small groups communicating online
Police say children even as young as 14 are involved, who are often vulnerable, disenfranchised from society, or perhaps with mental health problems
While police are now working alongside MI5 to use the "full might of the UK counter-terrorism machine", the message is for society to stay vigilant.
Mr Basu said: "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.
"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" are due to be sentenced in London and Leeds 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."