An anti-extremism charity says it will take legal action against Nigel Farage unless he publicly apologises for branding their organisation "violent" and "extremist".
Hope not Hate said the comments, made in an interview with LBC radio during a tirade against the husband of murdered Labour MP Jo Cox, were an "outrageous attack" on the organisation and its supporters.
It said in a statement: "Our lawyer has just sent Farage a letter demanding he retracts and publicly apologises or we will begin legal proceedings against him."
The row developed after Farage sent a tweet on Tuesday morning (20 December) claiming Angela Merkel was to blame for the recent suspected terrorist attack on a Christmas market in Berlin, which left 12 people dead and 48 injured.
Pointing to Merkel's "open-door" immigration policy following the refugee crisis in Syria, he tweeted: "Terrible news from Berlin but no surprise. Events like these will be the Merkel legacy."
The tweet was in-turn criticised by Brendan Cox, whose wife Jo was murdered by far-right fanatic Thomas Mair a week before the EU referendum in June.
He wrote: "Blaming politicians for the actions of extremists? That's a slippery slope Nigel."
During an LBC interview broadcast later that morning, Farage responded by accusing Cox of himself supporting extremists.
"He would know more about extremists than me, Mr Cox," he said. "He backs organisations like Hope not Hate who masquerade as being lovely and peaceful but actually pursue violent and very undemocratic means."
Farage also told LBC: "And I'm sorry Mr Cox, but it is time people started to take responsibility for what's happened. Mrs Merkel has directly caused a whole number of social and terrorist problems in Germany. It's about time we confronted that truth."
When the LBC host, Nick Ferrari, said Cox knew first-hand the consequences of extremism, Farage replied: "Yes, it's a terrible thing what happened, with the murder of his wife."
Farage's attack on Cox was quickly condemned by Labour MPs, including Tracy Brabin who tweeted: "Beggars belief... A new low for Farage."
Another Labour MP, David Lammy, tweeted: "Insulting the widower of a woman murdered by terrorists. A period of silence on your part would be welcome Nigel."
Hope not Hate, which describes itself as Britain's "largest anti-racist organisation", rejected Farage's accusations and said it has "a proud history of campaigning against extremism and hatred".
The charity, which is part funded by trade unions, regularly attacks far-right organisations and other extremist groups, saying in its mission statement that it's aim is to "defeat hate groups at elections and to build community resilience against extremism".
It was one of three organisations chosen by the Cox family to receive £2m (€ 2.38m, $2.47m) donated by members of the public following Jo Cox's death.
The charity has branded Ukip as "hardline" in the past, launched campaigns against the group during elections and often attacks the party's senior membership in its 'Purple Rain' blog.
Farage has regularly accused Hope not Hate members of disrupting his meetings and speaking events. In 2014, he told the BBC he was forced to hire bodyguards because of harassment from the group and from another organisation, Unite Against Fascism.
Hope not Hate at the time rejected Farage's claims as "ridiculous", saying it had never sought to disrupt his events.