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.The official threat level for international terrorism has stood at severe, meaning an attack is “highly likely”, for more than two years. Thomas Mair was sentenced to life for murdering Jo Cox MP in a Right-wing extremist attackCredit: HANDOUT Terror arrests of suspected Right-wing extremists and neo-Nazis more than doubled last year amid fears of a growing threat of political violence from far-Right groups, new Home Office figures show.A total of 35 people were arrested on suspicion of “domestic” terrorism in 2016, which security sources said was dominated by threats from the far-Right.The arrests followed only 15 for domestic terrorism the previous year and come after a warning from the Government’s terrorism watchdog that far-Right extremists now account for one-in-four of those reported to counter-radicalisation schemes. A report from counter extremism campaign group Hope Not Hate, last month concluded there was “a growing risk of violence and even terrorism from ever smaller but more extreme far-Right groups”.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. Figures also showed that one-in-three terrorism arrest suspects is now white, up from a quarter in 2015. The increase in domestic terrorism came as arrests for international terrorism including threats like Islamic State and al Qaeda fell slightly and arrests for Northern Ireland-related terrorism remained largely unchanged.A neo-Nazi group called National Action in December became the first extreme Right-wing group to be banned as a terrorist organisation.The anti-Semitic and white supremacist group had celebrated the murder of the Labour MP Jo Cox by Right-wing extremist Thomas Mair. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.