Her attacker reportedly shouted "Britain First" as he stabbed and shot her to death. And, though there is nothing to suggest any direct link with the group, the attack has shone a spotlight on the political party that has courted controversy.
The BNP off-shoot, founded just five years ago, has acquired a paramilitary style image: with countryside training camps for members and pledges to take “direct action” against “global Islamic jihad”.
It has appealed to former soldiers to join up – with images of guns and skulls on its website and the slogan "Isis hunting club" emblazoned on its merchandise.
On the day of Ms Cox's death, the front of its site showed pictures from the group's recent "Activist Training Camp, where members practised fighting techniques and posed in camouflage clothing.
The site is also littered with anti-immigrant and negative stories about Muslims. Examples include:
Headlines under the site's 'News' section include:
Some fear that the group and its poisonous right-wing ideology may have influenced Ms Cox’s killer.
The group's leader, Paul Golding, has claimed that the attacker could have shouted 'You need to put Britain first'.
Over the last year she has been at the forefront of the campaign to help Syrian refugees and pressurise the Government into accept Britain’s "fair share" of asylum seekers.
Founded just five years ago – by two former BNP members – Britain First has been highly adept at appealing to the most extreme elements of Britain’s right wing.
The group even has a wing called the “Britain First Defence Force”, which has attracted widespread condemnation for staging marches the group calls “Christian Patrols” in which members in green uniforms carry white crosses, while shouting anti-Islamic slogans.
They have also been involved in Mosque “invasions” and have staged protests outside the homes of alleged Islamists.
At one stage its members – complete in military style uniforms – offered to provide personal protection to the Ukip leader Nigel Farage. The offer was ingored by Farage who has no connection to the group, and has actively sought to distance himself and Ukip from it.
But the group has also had some success nationally, and garnered a large online following. On Facebook the group’s page has 1.4 million likes, meaning the far-right group has more followers on the social network than any political party in the UK. The Conservatives currently have the second largest following with 550,000 followers, while Labour has 480,000.
The group boasted of reaching 1 million supporters on its website, claiming the milestone indicated “genuine popular support of its ideals, policies and views”. However, this has been largely attributed to the frequent populist and sentimental posts on the page, many of which have little to do with the group’s core message.
At the 2014 European elections, founder members Dowson and Golding stood as Britain First candidates in Wales and Scotland. The group urged voters in England to vote for UKIP.
During the campaign in Wales, the group used images of murdered soldier Lee Rigby in their campaign, using the phrase “remember Lee Rigby” on ballot papers. The electoral commission subsequently apologised to the dead soldier’s family.
Speaking about the group, Lee Rigby’s mother said: “Their views are not what Lee believed in and has no support from the family.”