Barcroft Media via Getty Images Paul Golding and Jayda Fransen from the far-right group Britain First, a pseudo-political activist party that calls itself a "patriotic resistance" that opposes Islam, political correctness and the EU.

The killing of lawmaker Jo Cox this week raises concerns about the far-right fringes of a national debate on immigration and sovereignty in the lead-up to Britain's referendum on the European Union.

The 41-year-old member of Parliament was fatally shot and stabbed on Thursday on the streets of Birstall, northern England. The former aid worker and rising political star was a passionate advocate for immigration. She backed Britain remaining in the EU as the June 23 vote loomed on whether the country should exit Europe's politico-economic union, a move commonly known as a "Brexit."

Thomas Mair, the 52-year-old man charged with her murder, gave his name as “death to traitors, freedom for Britain” when he appeared in a London court on Saturday.

Prosecutors said Mair told police he is a "political activist" and that officers found far-right materials in his house. Mair reportedly had contacts with far-right groups in South Africa and the U.S. in the past. His family said he has a history of mental illness.

Witnesses heard Cox's killer yelling, “Britain first, Keep Britain independent, Britain always comes first,” during the attack. 'Britain first' is a popular slogan among English nationalists and also the name of an active anti-EU, anti-Islam political organization.

Britain First leader Paul Golding denied the group had any connection to Cox's murder in a video statement posted to its Facebook page and website soon after the attack. "We hope that this person who carried it out is strung up by the neck on the nearest lamppost," Golding said in the video.

While it is not yet clear whether Mair had any involvement in the group or other far-right organizations in the U.K., Cox's killing has drawn attention to political extremism in Britain, amid increasingly heated rhetoric in the lead-up to Thursday's EU referendum.

Neil Hall / Reuters British lawmaker Jo Cox was killed on the streets of northern England on Thursday. The man charged with her murder gave his name in court as “Death to traitors, freedom for Britain.”

What is Britain First?

Britain First was founded in 2011 and emerged out of similar far-right British nationalist groups, including the English Defence League and the British National Party. The pseudo-political activist party calls itself a "patriotic resistance" that opposes Islam, political correctness and the EU.

Although Britain First is on the very fringe of U.K. politics, the organization has grown a significant social media following through far-right memes and viral anti-immigration propaganda videos on the refugee crisis. One video shows leader Paul Golding mockingly questioning refugees and migrants in Calais on where they got their mobile phones, trying to portray them as wealthy and not in need of asylum.

In an attempt to gain more publicity, Golding ran for Mayor of London this year and received just over 1 percent of the vote. When London elected Sadiq Khan as its first Muslim mayor in May, Golding turned his back to the podium during the victory speech.

Britain First also tried to exploit the memory of British soldier Lee Rigby in promotional materials during its campaign. Rigby was killed in 2013, after Islamic extremists attacked him near a military barracks in Woolwich, southeast London. The soldier's family condemned the group, stating that "Britain First's views are not what Lee believed in and they have absolutely no support from his family."

The far-right group had become something of a punchline for U.K. news organizations, due to its gaffes and bizarre media output. On a trip to Northern Ireland, members of Britain First mistook a town hall for a mosque and posed in front of it as part of their propaganda. The U.K. Independence Party, a more mainstream anti-EU and anti-immigration political organization, has also shunned advances from Britain First. UKIP says the extremist group is trying to associate itself with the party to gain support.

Beyond the bluster and rhetoric of Britain First, members have also been involved in militant anti-Muslim activism. They've invaded mosques to harass imams and conducted "Christian patrols" in Muslim-majority neighborhoods to incite clashes as part of this strategy.

Carl Court/Getty Images Britain First mayoral candidate, Paul Golding, turns his back as Sadiq Khan, Labour's newly elected London mayor, gives his victory speech. Golding finished eighth out of 12 candidates.

Nationalists Stir Fears Of Immigration

While Golding and Britain First were quick to distance themselves from Cox's murder, her killing has come at a time when anti-EU, anti-immigrant sentiment and political tensions have come to the fore in Britain. As the referendum over a potential British exit from the EU gets closer, far-right political parties have made controversial and divisive appeals to leave the union -- many playing to ethno-nationalist sentiment.

Nationalist parties such as UKIP have stirred up fears about immigration by casting refugees from Africa and the Middle East as potential terrorists or economic chancers, and conflating the refugees journeying through Europe, most of whom will never reach Britain, with EU citizens' migration.

The latest immigration figures show net migration in the U.K. was 330,000 last year, of which 184,000 were EU nationals. Some 41,500 people applied for asylum in Britain in the year leading up to March 2016, most of them from Iran, Iraq, Afghanistan, Pakistan and Eritrea.

Stefan Wermuth / Reuters Hours before Cox was killed, United Kingdom Independence Party leader Nigel Farage Farage unveiled a poster showing a long line of refugees under the banner "Breaking Point."

'Breaking Point'

On Thursday morning, hours before Cox was killed, UKIP leader Nigel Farage unveiled a poster showing a long line of refugees under the banner "Breaking Point." Its captions blamed the EU for migration to the U.K. "As you will see from today's advert, the EU has failed us all," Farage wrote in a British newspaper on Thursday, calling it the largest advertising campaign in the party's history.

The image in the UKIP poster, which Getty Images photographer Jeff Mitchell took in Slovenia in October 2015, does show a crisis for the EU, but a very different one to that which Farage portrays. Hungary's nationalist government shut its border with Croatia that month, pushing thousands of migrants and refugees to journey through Slovenia, hoping to continue north to wealthier and more welcoming nations like Germany and Sweden.

Mitchell's photo captures Slovenian police escorting them to a refugee camp, as the country grappled with the consequences of the Hungarian backlash to the European refugee crisis. It highlights the fall-out of countries shutting borders and acting alone, not the opposite -- as Farage suggests.

"It is always uncomfortable when an objective news photograph is used to deliver any political message or subjective agenda, however the image in question has been licensed legitimately," Getty Images said in a statement to the WorldPost.

Critics of UKIP also pointed out that their newest campaign bears a strong resemblance to Nazi propaganda.

Your new poster resembles outright Nazi propaganda, @Nigel_Farage. Thanks to @brendanjharkin for pointing it out. pic.twitter.com/Rd89XZSvfD — Connor Beaton (@zcbeaton) June 16, 2016

UKIP's Farage also suggested last month that there could be violence if immigration isn't controlled and citizens feel they don't have a political voice. He also claimed that the Labour Party backed immigration to "rub our noses in diversity."

Anti-Immigrant Rhetoric Spreads

Other parties have also played on fears of foreigners and immigration, including the far-right British National Party, which circulated inaccurate statements that more than 80 million Turkish people will "pour" into Europe as a result of Merkel's EU policies.

Even ﻿mainstream British politicians have been accused of anti-immigration scaremongering. The Vote Leave campaign, led by British justice secretary and Conservative MP Michael Gove, in March published a list of murders and rapes it said EU citizens had committed in Britain.

In his video defending Britain First against any connection to Cox's attacker, Golding inadvertently summed up the kind of rhetoric that has become common during the Brexit campaign.

"It's the name of our party, yes, but I've heard UKIP people, I've heard Nigel Farage, everyone say it's time we put Britain first. It's the kind of language that's been utilized during this referendum campaign."

British politicians have warned of the dangers of such escalating rhetoric in the wake of Cox's death.

“Unless we strive for a culture of respect to replace a culture which does too little to challenge prejudice, we will be learning nothing from what happened to Jo,” former British Prime Minister Gordon Brown wrote in the Guardian after Cox was killed.

Charlotte Alfred contributed reporting.

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