Although cancer is a leading cause of death in the European Union, around 40% of cases are preventable. The European Code Against Cancer (ECAC) was developed to inform citizens about key cancer-risk-reducing actions. This study aimed to identify effective ways to present the 5th edition of the code (ECAC5) to optimise awareness of cancer risks in all socioeconomic groups. Using a 2 × 3 × 2 factorial design, 10 027 participants from eight countries were randomised online to receive ‘no message’ or one of 10 ECAC5 formats differing in message content (cancer risks: present/absent), length of message on cancer prevention actions (longer/shorter/absent) or format (text-only/text with images). The primary outcome was awareness of 16 avoidable cancer risks. Overall mean number of risks recalled was 2.40 (standard deviation: 1.72; range 0–14). Recall was highest when messages included risk information. Adding prevention messages to risk information did not improve risk factor recall. Message length and images had no significant impact. Effects were similar across levels of education and countries. Combined information about risk factors and preventive actions has the potential to equitably increase citizens’ very low cancer prevention awareness. How this awareness might change over time or lead to behaviour change is unknown and should be the focus of future evaluations.