The police and the military were warned eight months ago that the banned extremist group National Action was trying to infiltrate the Armed Forces, it has emerged.
Five men, four of whom are serving soldiers, are currently being held on on suspicion of terrorism offences after a "pre-planned and intelligence-led" by the authorities.
But as the questioning of the men continued, it emerged that a leading anti-extremism group has repeatedly highlighted the fact that National Action - which was banned by the Government in December 2016 in the wake of the murder of MP Jo Cox - was still active.
A blog titled "A look behind the scenes in National Action", posted in December 2016 just before the ban on National Action came into force, detailed training and "hate camps" that had been organised by National Action.
The Hope not Hate blog pointed out: "A number of National Action supporters/members have decided however to apply to join the British Army."
In January the charity then named and photographed a man who they identified as a National Action member who had sucessfully signed up to the Army.
The charity's website also named two members who were applying for the Army in a seperate profile and a blog post in April - months after National Action became the first right-wing organisation proscribed by the Home Office.
Police forces and the military were sent links to the blogs by concerned members of the public, it is understood.
Hope not Hate also detailed how "violence and acting like paramilitaries became more and more important to the group".
Matthew Collins, head of research at Hope not Hate, said: "Nothing was done about it.
"National Action are far more determined, far more sophisticated, than other groups. They are not just Hitler admirers, they are at the point where they admire all kinds of genocide."
Dr Paul Jackson, an expoert in neo-Nazi extremism from the University of Northampton, said that the group had adopted a "paramilitary style".
It comes as experts warn that far-right groups have increasingly attempted to align themselves with the military particularly in the wake of Lee Rigby's murder and the threats against soldier.
Dr Jackson said that the EDL had "idealised the army as heroes" and other groups saw themselves as having a "connection" to the armed forces.
Michael Fallon, the Defence Secretary, said that he was "very concerned" about news of the arrests but the extremist views are "absolutely nothing to do with the values and ethos of the armed forces".
Mr Fallon said that he could not comment on ongoing cases but said that the Armed Forces would root out any right-wing extremists.
He said: "My message is they will be rooted out and thats why that organisation has been proscribed by the Home Secretary, that is why membership of it is a criminal offence.
"It doesn't matter whether you are an extremist of the far left or an extremist of the far right, you have got values that have no place in our society.
"Indeed we welcome Muslims and people of other faiths into our Armed Forces, we are growing the number of people from those faiths in our Armed Forces because we want our Armed Forces to be able to reflect the diversity that is in our society."
West Midlands police yesterday continued to question a 22-year-old from Birmingham, a 32-year-old man from Powys, a 24-year-old from Ipswich and a 24-year-old from Northampton who had been arrested on Tuesday .
A fifth suspect was being flown back from the British Army base in Cyprus where he had been arrested.
Three of the men belong to the Royal Anglian Regiment and a fourth is a member of the Royal Electrical and Mechanical Engineers