ABSTRACT1. A revised classification scheme is described for standing waters in Britain, based on the TWINSPAN analysis of a dataset of aquatic plant records from 3447 lakes in England, Wales and Scotland, which is held by the Joint Nature Conservation Committee.2. Separate ecological descriptions of 11 distinct lake groups (A-J) are presented with summary environmental data, macrophyte constancy tables and maps showing their distribution. These lake groups include small dystrophic waters dominated by Sphagnum spp.; large, acid, upland lakes supporting a diversity of plant species, including Juncus bulbosus, Littorella uniflora, Lobelia dortmanna and Myriophyllum alterniflorum; low-altitude, above-neutral lakes with a high diversity of plant species, characterized by the presence of Potamogeton spp., Chara spp. or water-lilies and other floating-leaved vegetation; and coastal, brackish lakes, with macroalgae.3. The Plant Lake Ecotype Index (PLEX) is presented as an indicator of changing lake environments. PLEX scores reflecting the new classification scheme have been developed for individual plant species and lakes. Applications of the index are demonstrated.4. There is discussion of possible applications of the data collected and the resultant classification, in the context of the Habitats Directive, the Water Framework Directive and other conservation requirements.