Former UKIP leader Nigel Farage. BBC LONDON — An anti-fascism group is threatening to take former UKIP leader Nigel Farage to court for a "political smear" he made about them while criticising the widower of murdered MP Jo Cox.

Hope not Hate has written to Farage demanding he retracts and publicly apologises for claims he made during an appearance on LBC radio on Tuesday morning.

The organisation has launched a crowdfunding page where people can donate money to help cover the group's legal costs.

The uproar relates to comments Farage made about Brendan Cox's affiliation with the anti-racism group, which a spokesperson for the group described as a "political smear" in a statement emailed to Business Insider.

Earlier in the day, Cox, whose wife Jo was murdered by a far-right extremist earlier this year, responded to a tweet by Farage which suggested that German Chancellor Angela Merkel was to blame for Monday's attack in Berlin, where a truck was driven into a Christmas market.

Farage said the tragedy was "no surprise" and would be remembered as part of "the Merkel legacy."

Cox responded by stating that it was a "slippery slope" to blame politicians for the actions of extremists, an apparent reference to Farage's own previous behaviour.

Shortly after this exchange of tweets, which went viral on social media, Farage used his slot on LBC to launch an incredible attack on Cox, suggesting he is familiar with extremists because of his links to Hope Not Hate.

He said: "Well, he [Cox] would know more about extremists than me, wouldn't he. He backs organisations like Hope Not Hate, who masquerade as being lovely and peaceful, but actually pursue violent and very undemocratic means."

The group says its lawyers have written to Farage demanding he apologises and retracts his remarks, otherwise he will face legal action for alleged defamation. In the statement, a spokesperson for Hope Not Hate said:

"Nigel Farage's allegations against HOPE not hate on LBC today are a political smear, which is why our lawyers have written to Mr Farage demanding that he retracts and publicly apologises for his remarks, or face further legal action.

"HOPE not hate is a well-respected, civil society organisation whose more than 200,000 supporters come from all political persuasions. They are united by a common desire to combat racism and to do so using lawful, peaceful means.

"That Nigel Farage made his remarks in the context of a discussion about Jo Cox, who was so brutally murdered earlier this year, makes them all the more poisonous and hateful.

As is well known, HOPE not hate was one of three entities chosen by Jo’s widow, Brendan Cox, as the recipient of donations from the public who wished to show their solidarity with the family."

Hope Not Hate did not reveal how much money they seek to raise when Business Insider spoke to one of their spokespeople on Tuesday afternoon. We have also contacted Farage's office for comment.