This species is named after the Isle of Purbeck in south-east Dorset where it is locally abundant on scrubby heathland. Its historical range also extends eastwards to the Isle of Wight and Hampshire, as far as the southern edge of the New Forest. In recent decades it has also been found in scattered locations in Devon and parts of central and eastern Ireland as well as Normandy in France. It can be recognised by its white flowers with large white or pale pink concave petals, ovate terminal leaflet, dark-margined sharply serrate leaflets and hairy stem. In many respects it is identical to Rubus leptothyrsos, a predominantly Scottish bramble.
Rubus purbeckensis is a low-arching species which forms low bushes or grows up through gorse scrub. The panicle is narrowly pyramidal, with a congested head of flowers at the apex. There are a few trifoliate leaves lower down and often several small single leaflets just below the main part of the inflorescence. The rachis is slightly flexuose, green or reddish, thickly pubescent, with long slender declining prickles. Very short-stalked glands and patent prickles occur on the pedicels.
The upper panicle leaflets are a matt yellow-green to dark green colour and shortly pubescent above, ovate or somewhat rhomboid in shape, plicate, with a short or sometimes long apex. The leaflets are sharply and narrowly serrate, giving a finely jagged and crisped appearance, made more distinctive by hairs showing between the teeth and a dark reddish-brown tinge around the margin.
Flowers are relatively large, 2.5-3cm diameter, with white or very pale pink ellipitcal petals 12-15 x 8-9mm, which are sometimes toothed or apiculate at the apex and often distinctly convcave. Stamens are dense and much longer than the styles, with white filaments and glabrous to hairy anthers (one specimen examined had glabrous and very hairy anthers in the same inflorescence). Styles are pale green; young carpels pilose (may also be glabrous); receptacle hairy. Sepals are densely pubescent with a few short-stalked glands and acicles, narrowly pale-bordered, with long finely pointed tips; reflexed soon after the flowers open.
Leaves are usually 5-foliate (sometimes 3-foliate), with the leaflets contiguous to slightly imbricate. The terminal leaflet is ovate to broadly elliptical, with an emarginate to subcordate base and an acuminate apex; usually dark green in colour. Leaflets are often moderately plicate like the panicle leaflets and often also have a dark-brown tinged, deeply and irregularly serrate (i.e. rather jagged) margin.
Leaflets are greenish felted below, both on the stem and in the panicle.
The stem is bluntly angled with flat or rounded sides, dull purplish brown, usually shortly pubescent with no stalked glands. The prickles are shorter to about the same length as the width of the stem, relatively slender, with a red base and a yellow tip.