Abstract: We investigate the region around the Planck-detected z = 3.26 gravitationally lensed galaxy HATLAS J114637.9-001132 (hereinafter HATLAS12-00) using both archival Herschel data from the H-ATLAS survey and using submm data obtained with both LABOCA and SCUBA2. The lensed source is found to be surrounded by a strong overdensity of both Herschel-SPIRE sources and submm sources. We detect 17 bright (S870 > ?7 mJy) sources at >4? closer than 5 arcmin to the lensed object at 850/870 ?m. 10 of these sources have good cross-identifications with objects detected by Herschel-SPIRE which have redder colours than other sources in the field, with 350 ?m flux >250 ?m flux, suggesting that they lie at high redshift. Submillimeter Array (SMA) observations localise one of these companions to ?1 arcsec, allowing unambiguous cross identification with a 3.6 and 4.5 ?mSpitzer source. The optical/near-IR spectral energy distribution of this source is measured by further observations and found to be consistent with z > 2, but incompatible with lower redshifts. We conclude that this system may be a galaxy cluster/protocluster or larger scale structure that contains a number of galaxies undergoing starbursts at the same time.

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