OBJECTIVE: To explore on-package formula messaging with reference to legislation and government-issued guidance in Great Britain (GB). DESIGN: Formula products were identified, pictures of all sides of packs collated and on-package text and images were coded. Compliance with both GB legislation and guidance issued by the Department of Health and Social Care (DHSC) was assessed. SETTING: All formula packs that were available for sale over the counter in GB between April and October 2020. PARTICIPANTS: Formula packs (n 71) including infant formula, follow-on formula, growing-up formula and specialist formula were identified, coded and analysed. RESULTS: In total, 41 % of formula packs included nutrition claims, and 18 % included health claims that may be considered non-permitted, according to DHSC guidance. Additionally, 72 % of products showed images considered 'non-permitted'. Breast Milk Substitute (BMS) legislation states infant and follow-on formula packs should be clearly distinguishable but does not provide criteria to assess similarity. Based on DHSC guidance, 72 % of infant and follow-on formula packs were categorised as showing a high degree of similarity. Marketing practices not covered by current legislation were widespread, such as 94 % of infant formula packs including advertisements for follow-on or growing-up formula. CONCLUSIONS: Text and images considered non-permitted according to DHSC guidance for implementing BMS legislation were widespread on formula products available in GB. As terms such as 'similarity' are not defined in BMS legislation, it was unclear if breaches had occurred. Findings support the WHO call for loopholes in domestic legislation to be closed as a matter of urgency.