Four extreme right-wing attack plots were foiled last year, Britain's most senior counter-terror officer has revealed.
Disclosing the figure for the first time, Assistant Commissioner Mark Rowley described the threat from far-right terrorism as "organised" and "significant".
He warned the overall terror threat is "considerable" with over 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 a lecture on Monday, Mr Rowley 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."
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 added that a further four extreme right-wing inspired plots were prevented.
Four extreme right-wing attack plots were foiled last year
Ten conspiracies of an Islamist nature have been stopped since last March
More than 600 investigations encompassing Islamist, extreme right-wing and other motivations are on the go at any one time
At least 20,000 individuals who have previously featured in inquiries are under review
The probes are focusing on more than 3,000 subjects of interest
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".
Speaking to the media prior to his speech, the senior officer 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".
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.