This study evaluated if injuries and mortalities among discarded European plaice (Pleuronectes platessa) are reduced by the European Union’s legislation for owners of > 221-kW double-rigged beam trawlers to use a ‘flip-up’ rope that limits large stones (>25 cm in diameter) and unwanted debris from entering trawls. During eight commercial trips comprising 197 beam-trawl deployments onboard five Belgian vessels during 2019 and 2020, two scientific observers assessed the reflex responsiveness, injuries and immediate mortalities of 3191 European plaice from 244 sampled codends, and the fate (delayed mortalities) of 268 specimens that were tagged, transferred and monitored for at least 21 days in captivity. Immediate on-board mortality was positively associated with physical injury, the total weights of catches and stones, and the proportion of injury-inducing elements among the unwanted catch. Delayed mortality was positively associated with injuries and impaired reflexes and elevated seawater temperature. A detailed, simultaneous catch comparison was done in December 2020 with one of the double-rigged beam trawls equipped with a flip-up rope and the other without. The odds of immediate and delayed mortalities were similar with or without the flip-up rope (odds ratio of 1.19 [0.90–1.97] and 1.22 [0.81–3.02] for the two trips assessing immediate mortality and 1.03 [0.52–2.11] for the trip assessing delayed mortality). Reducing catch weights and eliminating stones should reduce the injury and mortality of discarded plaice, but the performance of the legislated flip-up rope needs to be improved.