British police foiled four extreme right-wing terror plots in 2017, according to the country's chief counter-terrorism officer, warning of a "significant" threat.
"The right-wing terrorist threat is more significant and more challenging than perhaps the public debate gives it credit for," Mark Rowley said in a speech late on Monday.
"Right-wing terrorism wasn't previously organised here," said Rowley, who is stepping down this year.
He added: "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."
Britain was hit by five terror attacks last year, four Islamist-inspired and one extreme right-wing.
In 2016, a neo-Nazi sympathizer assassinated British MP Jo Cox in her constituency in northern England.
Investigations against the extreme far-right in Britain have focused in recent months on National Action, a group defined by Rowley as a "homegrown, white supremacist, neo-Nazi terror organisation".
The group was banned under terrorism laws in 2016.
Rowley also said that the children of convicted terrorists should be taken away from them.
"The family courts and social services now routinely wrestle with child protection and safeguarding cases arising out of terrorism and extremism," he said.
"However, we still see cases where parents convicted of terrorist-related offences, including radicalisers, retain care of their own children."