These two species can often be found flying at the same time of year and can be found in the same trap together.
Separation is best done by looking at the white lines in the black areas at the rear of the upper-side of the hind-wing close to the body.
This is best checked by CAREFULLY handling the moth and GENTLY extending one wing to see the area involved as when the wing is folded this is not visible.
In Swallow Prominent there is an obvious white line PARALLEL to the rear edge of the wing. In Lesser Swallow Prominent this is PERPENDICULAR (at right angles) to the rear edge of the wing. This is illustrated in the photos of the set specimens below.