In February, David Anderson QC, then the Government’s independent terror law watchdog, said far-Right terrorism could be as dangerous as Islamist violence.

Extremists were also increasingly seeking to “feed off the tension” caused by Islamist terror to plan violence of their own, he warned.

He said: “The threat from extreme Right-wing terrorism in the UK is currently fragmented but the massacre perpetrated by Anders Breivik in Norway is a warning against underestimating the threat.

“Both the Government and the courts treat the threat with the seriousness it deserves. Extreme Right-wing ideology can be just as murderous as its Islamist equivalent. A sophisticated network is not a prerequisite for mass slaughter.”

A 17-year-old member of National Action was last month given a three-year youth rehabilitation order after building a homemade pipe bomb.

The teenager from Bradford, who cannot be named, had also praised Mair online.

The total tally of terror arrests, 260, was down by eight per cent on 2015. International terrorism, such as suspects linked to Islamic State in Iraq and Levant (Isil), accounted for 203 of the arrests.

Earlier this week it was revealed that UK security services had foiled 13 potential attacks in less than four years, while counter-terror units were running more than 500 investigations at any time.