Save the Badger, a campaigning organisation run by charity Secret World Wildlife Rescue, had run a series of adverts encouraging people to challenge the planned badger culling trial in west Wales.
The Farmers' Union of Wales complained about the content of these adverts and the Advertising Standards Authority upheld nine of those complaints.
The advertisements, published in May this year, encouraged members of the general public to oppose badger culling, and called on them to write to the Welsh Assembly Government and the rural affairs minister.
But the ASA ruled that claims made in the advertisements breached either truthfulness, substantiation and matters of opinion codes on nine occasions.
These claims included false assertions that the proposed badger cull in west Wales had "no scientific justification" and that it would "exterminate a native breeding species".
FUW bovine tuberculosis spokesman Brian Walters welcomed the ruling. "This ruling clearly shows that Save the Badger is either incapable of understanding basic scientific principles regarding badgers and TB, or that they are prepared to mislead the general public in order to try and stop badger culling going ahead," he said.
"Whichever is true, they have shown utter contempt for the people of Wales, and are likely to have misled thousands of people into believing their false propaganda and campaigning against badger culling."
Save the Badger has been told by the ASA that the advertisements should not appear again in their current form. Brian Walters suggested the charity should go further by publishing a public apology.