The Large White is Britain's largest white butterfly with a wingspan of 58-63mm. Adults have broad black or dark grey tips on the upperside of the forewings and two black spots on the underside. The underside of the hindwings are plain yellow. Females are slightly larger than males and have black spots on the upper surface of the forewings. Summer generated adults have blacker markings than those of spring adults. The larvae are brightly coloured and easily seen, which serves to warn predators of the irritant and poisonous mustard oils they have concentrated from their foodplants.

The larvae have a a strong preference for feeding on cultivated varieties of Brassica oleracea such as cabbages and Brussels sprouts, also varieties of Oil-seed Rape, nasturtium, wild mignonette, and sea kale. Wild species of foodplants are not important for this species.

Large Whites may be seen in any location throughout Britain and Ireland, even on the tops of hills and mountains. Most adults are seen close to their breeding area, in gardens, allotments, and fields where brassica crops are grown. They may congregate in large numbers in fields of oil-seed rape.

Because of the damage their larvae inflict on brassica crops they are not the most popular of summer visitors to gardens, allotments and fields.