Benjamin Hannam, 22, who had worked as a probationary police constable with the London Metropolitan Police, was found guilty earlier this month of belonging to National Action. The group was outlawed after it praised the murder of Jo Cox, a female member of parliament who was killed in a street attack in 2016 by a Nazi-obsessed loner.
Hanman was also convicted of lying on his police application forms and owning terrorism documents. He had previously admitted possessing indecent images of a child.
Hanman, who was dismissed from the force after his conviction, was sentenced on Friday to four years and four months in jail at London's Old Bailey court.
"Benjamin Hannam lied on his police application and vetting forms in an attempt to hide his dangerous and racist beliefs," said Jenny Hopkins, Head of the CPS Special Crime and Counter Terrorism Division.
Hannam was first known to have become involved with National Action in early 2016, six months before it became the first far-right group to be outlawed in Britain since World War Two.
But he continued with his involvement in it, and with its offshoot NS131 after the government proscribed it. He lied about having any involvement with a far-right group on his application to join the police in 2018.
Detectives discovered his links in February 2020 following the leak of a database of members of a far-right online forum, Iron March.
At his home, officers found a notebook referring to the far-right group, a guide on how to use knives and weapons, and the manifesto of Norwegian mass killer Anders Behring Breivik, who killed 77 people in 2011 in Norway's worst peacetime atrocity.Detectives found Hannam had attended National Action meetings in pubs, taken part in training events, and appeared in online videos after the group was banned, but that there was nothing to suggest he had continued contact after joining the police.