  We report the discovery of an extended globular cluster in a halo field in
Centaurus A (NGC 5128), situated $\sim 38\kpc$ from the centre of that galaxy,
imaged with the Advanced Camera for Surveys on board the Hubble Space
Telescope. At the distance of the galaxy, the half-light radius of the cluster
is r_h ~ 17pc, placing it among the largest globular clusters known. The faint
absolute magnitude of the star cluster, M_(V,o)=-5.2, and its large size render
this object somewhat different from the population of extended globular
clusters previously reported, making it the first firm detection in the
outskirts of a giant galaxy of an analogue of the faint, diffuse globular
clusters present in the outer halo of the Milky Way. The colour-magnitude
diagram of the cluster, covering approximately the brightest four magnitudes of
the red giant branch, is consistent with an ancient, i.e., older than ~8 Gyr,
intermediate-metallicity, i.e., [M/H] ~-1.0 dex, stellar population. We also
report the detection of a second, even fainter cluster candidate which would
have r_h ~ 9pc, and M_(V,o)=-3.4 if it is at the distance of NGC 5128. The
properties of the extended globular cluster and the diffuse stellar populations
in its close vicinity suggest that they are part of a low mass accretion in the
outer regions of NGC 5128.
