The Advertising Standards Authority delivered its judgement in response to complaints made by the Farm-ers Union of Wales.
The union accused the RSPCA of making misleading claims in a series of high-profile newspaper advertisements designed to solicit public support against possible badger culls.
As a result, the government's consultations on anti-bovine TB measures may be flawed, said FUW president Gareth Vaughan.
He added: "The ASA ruling should serve as awarning to all pressure groups that they cannot twist the truth to subvert a public consultation process for
their own blinkered ends."
Celebrities including actress Joanna Lumley, author Jilly Cooper, ex-Spice Girl Mel C and former newsreader Angela Rippon supported the RSPCA campaign.
According to UK rural ministry Defra, it received 47,472 responses to the consultation, most of which were campaign responses prompted by, and supportive of, the RSPCA stance.
The FUW's complaint centred on the charity's assertion that cattle-to-cattle transmission of bovine TB (bTB) was the main conduit for the disease, not badger-to-cattle transmission.
In its adjudication, the ASA said: "We considered the claim did not reasonably provide
readers with an indication of the caution and uncertainty among scientists and government advisers surrounding the relative importance of the two factors in bTB transmission."
The RSPCA acknowledged that badger-to-cattle transmission was a factor in the spread of bTB, and had simply intended to alert readers to the fact that the issue of bTB was not straightforward.