This study introduces social norm theory to mis- and disinformation research and investigates whether, how and under what conditions broadsheets’ accuracy norm violation in political journalism becomes contagious and shifts other news media in a media market towards increasingly violating the accuracy norm in political journalism as well. Accuracy norm violation refers to the publication of inaccurate information. More specifically, the study compares Swiss and UK media markets and analyses Swiss and UK press councils’ rulings between 2000 and 2019 that upheld complaints about accuracy norm violations in political journalism. The findings show that broadsheets increasingly violate the accuracy norm the closer election campaigns approach to election dates. They thereby drive other news media in a media market to increasingly violate the accuracy norm as well. However, this holds only for the UK media market but not for the Swiss media market. Therefore, the findings indicate that the higher expected benefits of accuracy norm violation that exist in media markets characterised by higher competition outweigh the higher expected costs of accuracy norm violation created by stronger press councils’ sanctions, and, thereby, facilitate contagious accuracy norm violation in political journalism during election campaigns.