The killing of Jo Cox, the Labour MP, has prompted renewed fears of nationalist violence just as British groups normally regarded as extremist have been trying to make common cause with mainstream parties in the EU referendum campaign.

In recent years several far-right parties across Europe — including the British National Party under the leadership of Cambridge-educated Nick Griffin — have tried to make themselves more respectable and broaden their appeal. The BNP hit its peak in the 2009 European elections, when it secured 6.2 per cent of the vote and two seats in the European Parliament, including one for Mr Griffin.

However, this was followed by a rapid decline and internal splits, with the party losing support to the UK Independence party and a group called Britain First. The latter, founded by ex-BNP members, is described by the campaign group Hope Not Hate as “probably the only [UK] group experiencing growth on the far-right”.

Hard-right groups in the UK often reveal their extremist views overtly. For instance, members of the English Defence League, an anti-Muslim group prominent in 2009-11 for its provocative street protests, expressed admiration for Anders Behring Breivik, who killed 77 people in Norway in 2011.

But the EU referendum campaign has afforded the far-right the chance to make common cause with the anti-Europe, anti-immigration Ukip and the Eurosceptic right of the Conservative party — including, to the horror of those parties, by gatecrashing pro-Brexit events.

An EDL organiser was pictured in May alongside Nigel Farage, Ukip leader. In one image, Andrew Edge, who in 2013 was sentenced to 21 months in jail for violent disorder after unrest at a protest, poses with Mr Farage behind a Ukip pro-Leave placard.

Ukip said the photographs were taken after an event involving “hundreds of people”, adding that it could not be expected to run a profile of everyone who attended. “There is no association whatsoever [between Ukip and the EDL],” a spokesman said. The spokesman added that Mr Farage had himself faced security risks because of attempts to link him with the far right.

Vote Leave, the official Brexit campaign fronted by the senior Conservative Boris Johnson, demanded in May that Mark Collett, former chair of the BNP youth wing, stop using its official campaign materials. A spokesman for Vote Leave told the BBC: “Mark Collett and his associates are not part of our campaign.”

Mr Collett rejected the demand, saying on Facebook: “This is a pivotal moment in British politics and a real way nationalists can campaign for something worthwhile.”

© Reuters

At times, the rhetoric of the far-right has been hard to distinguish from that of the mainstream Leave groups. Last week, the BNP emailed supporters warning that 80m Turks were set to “pour into Europe”. The official Leave campaign made an almost identical point in a leaflet it sent to voters, while Mr Farage’s poster depicted a line of Syrian refugees has been highly controversial, with Michael Gove, justice secretary and another leading Eurosceptic, saying he “shuddered” when he saw it.

Asked on Sunday by ITV’s Robert Peston whether, following the murder of Cox, he felt responsible for stoking hatred, Mr Farage said he was “a victim of it”. He rejected criticism of the poster and warned of terrorists entering Europe pretending to be refugees, insisting Britain would be “safer taking back control”.

The leadership of the BNP and of Britain First have condemned the killing of Cox. But on far-right message boards over the weekend, reaction was mixed. On Stormfront, a prominent white nationalist forum, some praised the attack, while others saw it as counter-productive to the Leave campaign.

In Europe, perhaps the most successful example of a reinvented far-right party is France’s National Front led by Marine Le Pen. Ms Le Pen has managed to attract the support of more moderate voters by avoiding the anti-Semitic outbursts of her father and predecessor, Jean-Marie Le Pen.

Nonetheless, she has continued to rail against immigrants and Islam while courting working-class votes with promises to tackle the economic insecurities of globalisation. Recent polls have predicted she will come top in the first round of next year’s presidential election.

Ms Le Pen, like other European far-right leaders, swiftly sought to distance her party from the killing of Cox last week. Speaking as the first reports of the incident emerged, Ms Le Pen said it was “not very decent” to speculate that the attack was linked to the MP’s support for staying in the EU and her welcoming stance towards migrants.

Far-right leaders in Austria, the Netherlands, Germany and elsewhere have sought to emulate Ms Le Pen and break with — or disguise — their fascist past.

Weyman Bennett, joint secretary of the campaign group Unite Against Fascism, drew a distinction between “racist populists”, who oppose immigration and want to leave the EU but would never countenance violence, and “euro-fascists”, who embrace anti-EU sentiment as part of an effort to overthrow the democratic order in favour of white-supremacist rule.

“The campaign about leaving [the EU] means that if you are a fascist you now have a bridge into mainstream politics,” he said. “They have always felt that no one listens to them. Now they feel like they strike a chord.”

Additional reporting by Leila Haddou and Henry Mance.