Public concern around gambling advertising in the UK has been met not by government action but by industry self-regulations, such as a forthcoming voluntary ban on front-of-shirt gambling sponsorship in Premier League soccer. "Safer gambling" (harm prevention) adverts are one recent example, and are TV commercials which inform viewers about gambling-related harm. The present work is the first independent evaluation of safer gambling adverts by both gambling operators and a charity called GambleAware. In an online experiment, we observed the change in participants' (N = 2,741) Gambling Urge Scale (GUS) scores after viewing either: a conventional financial inducement gambling advert, a gambling operator's safer gambling advert, an advert from the GambleAware "bet regret" campaign, an advert from the GambleAware "stigma reduction" campaign, or a control advert that was not about gambling. Relative to a neutral control advert, GUS scores increased after viewing a financial inducement or an operator's safer gambling advert. In comparison to the neutral control condition, GUS score changes were similar after viewing a bet regret advert, but showed a significant decrease after viewing a stigma reduction advert. Those at higher risk of harm reported larger decreases in GUS after watching a bet regret or stigma reduction advert. Overall, this study introduced a novel experimental paradigm for evaluating safer gambling adverts, uncovered a potential downside from gambling operators' safer gambling adverts, and revealed variation in the potential effectiveness of charity-delivered safer gambling adverts.