The terrorist threat from extreme right-wing groups and individuals has grown significantly, with four right-wing plots foiled in the past year, according to the UK's most senior counter-terror police officer.
Scotland Yard assistant commissioner Mark Rowley, who retires in a fortnight's time, said neo-Nazi and far-right plots were becoming more organised and a matter of "real concern".
The figures are not routinely disclosed but Mr Rowley said it was important to make them public in order to "illustrate the growth of right-wing terrorism".
The assistant commissioner said: "The right-wing terrorist threat is more significant and more challenging than perhaps the public debate gives it credit for."
Pointing to a shift in the nature of the threat, he added: "Right-wing terrorism wasn't previously organised here."
He declined to give details of the four cases for legal reasons but said they "reflect a combination of the organised and the individual".
In a briefing to the media at Scotland Yard, followed by a lecture at the Policy Exchange think tank in central London, Mr Rowley said that Islamist related terrorism remains the most significant threat facing the UK.
As well as five attacks that hit Britain last year, police and MI5 intervened to disrupt an unprecedented number of suspected plots.
Ten conspiracies of an Islamist nature have been stopped since the Westminster attack in March last year, Mr Rowley said.
He warned the overall terror threat is "considerable" with more than 600 investigations encompassing Islamist, extreme right-wing and other motivations at any one time.
The probes are focusing on more than 3,000 subjects of interest, while security agencies must also keep at least 20,000 individuals who have previously featured in inquiries under review.
In his speech, he said: "A deeply concerning characteristic is how both far-right and also Islamist terrorism are growing, allowing each side to reaffirm their grievances and justify their actions."
His remarks underline the authorities' mounting concern over the far-right threat, which was laid bare in the murder of Labour MP Jo Cox in June 2016 and the Finsbury Park attack a year later.
In December 2016, National Action – which Mr Rowley described as a "homegrown, white supremacist, neo-Nazi terrorist organisation" – became the first extreme right-wing group to be banned under terrorism laws.
MI5 has been given an increased role in the response to domestic extremism, including far-right threats, which police have historically led on.
In a wide-ranging assessment of extremism and terrorism in Britain, Mr Rowley revealed that since the start of the conflict in Syria around 100 children have been safeguarded through the family courts.
He said: "The family courts and social services now routinely wrestle with child protection and safeguarding cases arising out of terrorism and extremism.
"However, we still see cases where parents convicted of terrorist-related offences, including radicalisers, retain care of their own children.
"I wonder if we need more parity between protecting children from paedophile and terrorist parents."
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Mr Rowley, who has led UK counter-terrorism policing for nearly four years, described the existing threat as acute, and called for a "whole society response".
As he prepares to leave his post, assistant commissioner Rowley said the tempo of terrorist activity was likely to remain extremely high for at least the next couple of years.