Population diets need to become healthier and more sustainable to limit their negative effects on health and environment. This study assessed the effect of a positioning intervention, in isolation and in combination with a labelling intervention, on the a) healthiness and b) environmental sustainability of food choices in an experimental online supermarket. Participants (n = 2220) were randomly assigned to one of five trials groups (control; healthier items shown earlier (“health position”); health position & nutri-score labels; environmentally sustainable products shown earlier (“eco position”); eco position & ecolabels) and completed a shopping task in an experimental online supermarket. Linear regressions showed that compared to control, mean scaled nutri-scores of shopping baskets were significantly lower (healthier) for health position (−2.30; 95 %CI: −3.07, −1.52) and health position & labels (−2.50; 95 %CI: −3.28, −1.72), with no significant difference between health position and health position & labels (−0.20; 95 %CI: −0.66, 0.25). The mean eco scores of shopping baskets were significantly reduced (more sustainable) for eco position (−24 %; 95 %CIs: −15 %, −31 %) and eco position & labels (−30 %; 95 % CIs: −22 %, 37 %) compared to control. The eco position & labels group had significantly lower mean eco scores (−8 %; 95 % CIs: −2 %, −14 %) compared to eco position. The positioning intervention improved health and environmental sustainability of food selections in an experimental online supermarket, with less robust evidence for a small additional effect of adding labels. There was no suggestion that adding labels that potentially make the positioning intervention more salient had any backfire effects.