The Large White butterfly, one of the two "cabbage whites", has a heavy black corner which goes all the way round the forewing edge (unlike that of the Small White which stops short). It is usually slightly larger than the Small White. The female (left) has two black spots on the topside forewing; the male (right) has no spots on the topside. Both have two spots on the underside.
The female also has a black streak on the topside (not always clear) as well as the two black spots. Wingspan is 55-65mm.
The male has no streak or spots on topside.