Abstract Introduction We evaluated how effective an advertising campaign that was piloted by Cancer Research UK in January/February 2018 was at promoting quit attempts by increasing awareness of the relative harms of e-cigarettes compared with smoking. Methods Adults (≥16 years, n = 2217) living in Greater Manchester (campaign region), Yorkshire & Humber and the North East of England (control regions) completed cross-sectional surveys immediately before and after the campaign period. Surveys measured socio-demographics, perceptions and use of e-cigarettes, and motivation and attempts to quit smoking. We tested interactions between time (pre, post) and region (campaign, control). Results 36.7% (95% CI 33.0%–40.6%) of those in the intervention region recognized the campaign. In the general population, interactions were nonsignificant for all outcomes except for perception of e-cigarettes as effective cessation aids, with smaller increases from pre- to post-campaign in the campaign (49.9% to 54.0%) compared with the control region (40.5% to 55.0%; odds ratio [OR] = 0.66, 95% CI .45–0.98). Among smokers, motivation to quit increased in the intervention region (44.0% to 48.0%) but decreased in the control region (40.5% to 21.5%; OR = 2.97, 95% confidence interval [CI] 1.25–7.16), with no other significant differences between regions over time. Bayes factors confirmed that nonsignificant results were inconclusive. Conclusions Compared with the control region, the campaign was associated with an increase in smokers’ motivation to quit but a smaller increase in adults’ perception of e-cigarettes as an effective cessation aid. There was insufficient evidence to determine whether the campaign affected other outcomes. Implications Past extended mass media tobacco control campaigns have been shown to change public attitudes towards smoking, improve motivation to quit smoking, and reduce smoking prevalence. Much less is known about shorter, targeted campaigns. Here we show that using mass media to communicate accurate information about the relative harms of e-cigarettes compared with smoking may be an effective strategy in increasing smokers’ motivation to quit. Moreover, even when only run for a month, such campaigns can reach a large proportion of the targeted population. Further research is needed to evaluate effects on quit attempts and success.