The Farmers' Union of Wales' animal health and welfare committee has accused the Charity Commission of bringing the nature of charities into disrepute by failing to confront the RSPCA over its threats to farmers.
The claim comes in the same week as RSPCA chief executive Gavin Grant attracted fresh criticism for suggesting in a BBC Panorama documentary that the organisation would "name and shame" those involved in a badger cull in England.
Both Mr Grant and RSPCA vice president Brian May have attracted harsh criticism over recent months following a string of threats against businesses in the English badger cull areas, and calls for a boycott of all UK milk if "badger-friendly" labelling did not appear on dairy products.
"There is no doubt whatsoever that these threats have brought the RSPCA into extreme disrepute amongst farmers and large numbers of the general public" said FUW animal health and welfare committee chairman Catherine Nakielny.
"The RSPCA do extremely important and worthwhile work but the increasing tendency towards extremist rhetoric and threats, which are tantamount to blackmail, undermines all that good work."
Dr Nakielny said that the FUW had written to the Charity Commission repeatedly over the years regarding the RSPCA's use of misleading campaigning and lurch towards extremism but that the commission seemed to have an "anything goes" policy.
"When an organisation becomes this radicalised and openly threatens an entire community then that not only brings the charity into disrepute but also undermines the very principles we all associate with charitable behaviour. It basically brings all registered charities and the body which governs them into disrepute."
In a recent letter, FUW president Emyr Jones told the commission that their "...response to [previous concerns raised by the FUW] and similar complaints have done nothing but encourage the charity to act with impunity, and I am therefore writing to ask, once again, that the Commission take decisive action in relation to the RSPCA's lobbying actions, which have now become extreme to the point of being both aggressive and threatening."
Mr Jones was also harshly critical of the RSPCA chief executive's recent claim in an interview with Farmers Guardian that the charity was "the oldest law enforcement agency still in existence in this country", telling the commission that the comment seemed to epitomise the way in which the RSPCA regarded itself as some form of surrogate police force, rather than a body focused on charitable activities.
Dr Nakielny said "The Charity Commission's response to our President's letter was typically benign towards the RSPCA's recent aggression, to the point where you wonder whether any organisation which is not engaged directly in party political activities might qualify for charitable status, despite our traditional notion that a charity should serve the public good in a reputable way.
"For those thousands who feel intimidated by the RSPCA's escalating threats, the actions of the charity are anything but charitable, yet the Charity Commission seems happy to give such extremism its blessing.
"If the Charity Commission believes that it does not have the powers to effectively deal with such aggression then it should seek to extend its powers."