  The over-dense environments of protoclusters of galaxies in the early
Universe (z>2) are expected to accelerate the evolution of galaxies, with an
increased rate of stellar mass assembly and black hole accretion compared to
co-eval galaxies in the average density `field'. These galaxies are destined to
form the passive population of massive systems that dominate the cores of rich
clusters today. While signatures of accelerated growth of galaxies in the SSA22
protocluster (z=3.1) have been observed, the mechanism driving this remains
unclear. Here we show an enhanced rate of galaxy-galaxy mergers could be
responsible. We morphologically classify Lyman-break Galaxies (LBGs) in the
SSA22 protocluster and compare these to those of galaxies in the field at z=3.1
as either active mergers or non-merging using Hubble Space Telescope ACS/F814W
imaging, probing the rest-frame ultraviolet stellar light. We measure a merger
fraction of 48+/-10 per cent for LBGs in the protocluster compared to 30+/-6
per cent for the field. Although the excess is marginal, an enhanced rate of
mergers in SSA22 hints that galaxy-galaxy mergers are one of the key channels
driving accelerated star formation and AGN growth in protocluster environments.

Astrophysics of Galaxies