We undertake the first detailed analysis of the stellar population and spatial properties of a diffuse substructure in the outer halo of M31 .
The South-West Cloud lies at a projected distance of \sim 100 Â kpc from the centre of M31 , and extends for at least \sim 50 Â kpc in projection .
We use Pan-Andromeda Archaeological Survey photometry of red giant branch stars to determine a distance to the South-West Cloud of 793 ^ { +45 } _ { -45 } Â kpc .
The metallicity of the cloud is found to be [ Fe / H ] = -1.3 \pm 0.1 .
This is consistent with the coincident globular clusters PAndAS-7 and PAndAS-8 , which have metallicities determined using an independent technique of [ Fe / H ] = -1.35 \pm 0.15 .
We measure a brightness for the Cloud of M _ { V } = -12.1 mag ; this is \sim 75 per cent of the luminosity implied by the luminosity-metallicity relation .
Under the assumption that the South-West Cloud is the visible remnant of an accreted dwarf satellite , this suggests that the progenitor object was amongst M31 ’ s brightest dwarf galaxies prior to disruption .