  We present high-resolution imaging of a sample of 10 SCUBA submillimetre
galaxies, obtained using the ACS camera on the Hubble Space Telescope. We find
that the majority show compact, disturbed morphologies. Using quantitative
morphological classification, we find that at least 6 are classified as major
mergers. Simulations suggest that the morphological parameters are unlike local
spirals and ellipticals, but similar to those of local ULIRGs. Compared to
Lyman-break galaxies, the submillimetre-selected galaxies are on average more
asymmetric, but also significantly more concentrated in their light
distributions. This is consistent with a higher fraction of the stellar mass
being contained within a central spheroid component. Despite their different
morphologies, we find that submm galaxies are similar in size to luminous
(>~L*) Lyman-break galaxies, with half-light radii in the range 2.8 - 4.6kpc.
