BACKGROUND: Poor diet is a leading driver of obesity and morbidity. One possible contributor is increased consumption of foods from out of home establishments, which tend to be high in energy density and portion size. A number of out of home establishments voluntarily provide consumers with nutritional information through menu labelling. The aim of this study was to determine whether there are differences in the energy and nutritional content of menu items served by popular UK restaurants with versus without voluntary menu labelling. METHODS AND FINDINGS: We identified the 100 most popular UK restaurant chains by sales and searched their websites for energy and nutritional information on items served in March-April 2018. We established whether or not restaurants provided voluntary menu labelling by telephoning head offices, visiting outlets and sourcing up-to-date copies of menus. We used linear regression to compare the energy content of menu items served by restaurants with versus without menu labelling, adjusting for clustering at the restaurant level. Of 100 restaurants, 42 provided some form of energy and nutritional information online. Of these, 13 (31%) voluntarily provided menu labelling. A total of 10,782 menu items were identified, of which total energy and nutritional information was available for 9605 (89%). Items from restaurants with menu labelling had 45% less fat (beta coefficient 0.55; 95% CI 0.32 to 0.96) and 60% less salt (beta coefficient 0.40; 95% CI 0.18 to 0.92). The data were cross-sectional, so the direction of causation could not be determined. CONCLUSION: Menu labelling is associated with serving items with less fat and salt in popular UK chain restaurants. Mandatory menu labelling may encourage reformulation of items served by restaurants. This could lead to public health benefits.