At first sight you may think that this is a small white but if it is in your garden you need have no worries. The green-veined white (Pieris napi) is certainly a close relative of the small white but it feeds exclusively on wild crucifereae plants such as garlic mustard, cuckooflower, wild radish and so on so your greens are safe!
The green-veined white has two broods and the first flies throughout May. There will be a subsequent brood, here in the south at least, flying in August and September.The second brood isfond of other plants too, they especially go for thistles and members of the daisy family.
It is a problem positively identifying this butterfly in flight but as soon as it lands and closes its wings the green markings, that is the veins, on the underside of the wing can be seen from some way off. It is now quite a common butterfly; indeed I think, at times, it is now more common that the small white which is remarkable as the small white was once undoubtedly a very common butterfly.