The association of carbon isotope discrimination of grain (Δ13C) with yield performance under rain-fed and well-watered conditions was analysed using a doubled-haploid (DH) winter wheat population, derived from the cross between cvars Beaver×Soissons, within field experiments at two site-seasons. The aim of this work was to quantify associations between Δ13C and yield responses to drought and to test effects of major genes (the semi-dwarf genes, Rht-B1b, Rht-D1b, an awn suppressor gene, B1 and the 1BL.1RS wheat–rye chromosome translocation) segregating in the population for associations with Δ13C and drought performance. Carbon isotope discrimination, through its negative relationship with transpiration efficiency, may be used as a surrogate for this trait. Grain Δ13C was positively associated with grain yield under both irrigated and unirrigated conditions in each site-season and, overall, explained 0·34 of the phenotypic variation in grain yield amongst DH lines under drought and 0·14 under well-watered conditions. There was a positive association between specific leaf lamina N content (SLN) at anthesis and Δ13C under drought amongst DH lines in one site-season, suggesting higher SLN may confer increased stomatal conductance via higher photosynthetic capacity, hence increased grain Δ13C. Overall the Rht-D1b (semi-dwarf) lines had slightly higher Δ13C of grain (20·0‰) than the Rht-B1a/Rht-D1a (tall) group of lines (19·8‰). There were no significant differences between the Rht-B1b (semi-dwarf) or the Rht-B1b/Rht-D1b (dwarf) lines and the tall lines. Comparing their performance under irrigated and unirrigated conditions, the Rht groups of lines (Rht-B1b semi-dwarf, Rht-D1b semidwarf and dwarf and tall groups) responded no differently to drought for Δ13C. The Rht-D1b semi-dwarf lines had higher grain yield (9·50 t/ha) than the tall lines (8·76 t/ha), while the yield of the Rht-B1b semi-dwarf and dwarf lines did not differ significantly from the tall lines. In each site-season, the presence of the 1BL.1RS chromosome increased grain Δ13C (P<0·001), with an overall increase from 19·7‰ in the 1B lines to 20·0‰ in the 1BL.1RS lines (P<0·001). However, the 1BL.1RS and 1B lines responded similarly to drought. The effect of the presence/absence of awns on grain Δ13C was not statistically significant in either site-season. Overall, the present results show that Rht-D1b confers higher Δ13C and grain yield, and the 1BL.1RS translocation confers higher Δ13C. This implies that modern UK wheat cultivars may have lower water-use efficiency during the grain filling period than their predecessors, and therefore may require more water to fulfil their yield potential.