A tidal barrage across the Severn Estuary (UK) was first proposed in the 1970s and similar ideas have re-emerged in recent years as the pressure for sources of renewable energy increase. Claims that the barrage would deliver ecological and flood defence benefits based on the tidal power project at La Rance in France are examined. This analysis suggests that the range and scale of ecological and geomorphological impacts will be considerably more deleterious than has hitherto been documented. Monitoring of the Eastern Schelde tidal barrage, constructed to reduce flood events in Holland, provides important pointers about the possible effects of a barrage across the Severn Estuary. The published outcomes of detailed monitoring on the Eastern Schelde provide a robust analogue that suggests significant detrimental effects on nature conservation are likely and that some of the functional changes also have important implications for flood defence structures around the estuary and its tributaries.