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Digital wheat grain classification systems are primarily based on kernel image and texture features. These traits are highly environment-dependent, so sample-to-sample variation may affect results of grain classification, grading, or discrimination. The main aim of this study was to determine the intra-cultivar variation of wheat kernel size, shape, colour, endosperm hardness and selected chemical compounds (carotenoids, phlobaphenes and proteins) associated with kernel colour. In the study, four winter bread wheat cultivars: two self-pollinated 'Bogatka' and 'Batuta' and two hybrid cultivars 'Hybred' and 'Hymack' were grown in different environments (extensive and intensive cultivation regimes, and three growing locations) to create variation in the kernel image, hardness and chemical compounds to simulate the range of variations encountered in reality. Measurements of kernel colour and dimensions were conducted using digital image analysis, hardness and protein content were analysed using nearinfrared reflectance (NIR) spectroscopy, while carotenoids and phlobaphenes were examined using colorimetric assays. The results showed that kernel dimensions and shape were highly intra-cultivar constant, with the highest variation of thickness (up to 7.59%) and volume (up to 10.10%). Surface colour was also relatively constant, with saturation as the most variable feature (up to 11.44%). In contrast, endosperm colour revealing an approx. 2-3-fold higher variation of colour attributes than surface colour, and the highest variation was observed in the case of saturation of the colour (up to 19.67%). Among the tested chemical compounds, carotenoids were the least intra-cultivar variable (up to 14.39%), while phlobaphenes the most intra-cultivar variable (up to 31.87%). Grain hardness was the most intra-cultivar variable feature of the tested grain samples and reached the highest value of 49.02% for cultivar 'Hymack'. The genotype had the greatest effect on kernel colour (up to 50.49% of the source of variation) and carotenoids (52.97%). The environment affected mostly the grain proteins (82.20%) and hardness (72.61%). [ABSTRACT FROM AUTHOR]