Nigel Farage has attacked the husband of murdered MP Jox Cox over extremism claiming the widower backs organisations that pursue “violent and very undemocratic means”.

The former Ukip leader became embroiled in a row with Brendan Cox after he questioned a claim that the attack in the German capital was a result of Angela Merkel policies.

Writing on his Twitter account Mr Farage said the incident – in which at least 12 people have died and 48 injured when a lorry ploughed through a Christmas market – was “terrible news” but “no surprise”.

Mr Cox, whose wife was murdered by the right-wing terrorist Thomas Mair, shortly before the EU referendum, had replied saying that “blaming politicians for the actions of extremists” was a “slippery slope”.

But then, in an extraordinary step, the former Ukip leader then said in an interview with LBC: "Well, of course, he would know more about extremists than me, Mr Cox, he backs organisations like Hope Not Hate, who masquerade as being lovely and peaceful but actually pursue violent and very undemocratic means.

He continued: "I'm sorry Mr Cox, it is time people started to take responsibility for what's happened. Mrs Merkel had directly caused a whole number of social and terrorist problems in Germany, it's about time we confronted that truth."

Shortly after the incendiary interview, Hope Not Hate released a statement said they were aware of a “serious and potentially libellous” statement made by Mr Farage on LBC.

“We have no idea on what Mr Farage bases his outrageous comments,” the organisation added. “Hope not hate has a proud history of campaigning against extremists and hatred”.

“We will not be making any further comments until we have had the opportunity to consult with out lawyers.”

Chris Bryant, the Rhondda MP and former shadow Commons leader, said: "The sheer nastiness of Farage sometimes takes my breath away."

Chesterfield MP Toby Perkins added: "When your entire career has been built on hate, not hope, it perhaps shouldn't shock me, but Farage still sinks lower than I'd have believed."