We present first results from a deep (J = 18.7), wide-field (6.5deg2) infrared (ZY JHK) survey in the Upper Sco association conducted within the science verification phase of the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Cluster Survey (GCS). Cluster members define a sequence well separated from field stars in the (Z - J, Z) colour-magnitude diagram. We have selected a total of 164 candidates with J = 10.5-18.7 mag from the (Z - J, Z) and (Y - J, Y) diagrams. We further investigated the location of those candidates in the other colour-magnitude and colour-colour diagrams to weed out contaminants. The cross-correlation of the GCS catalogue with the Two-Micron All-Sky Survey data base confirms the membership of 116 photometric candidates down to 20 Jupiter masses as they lie within a 2σ circle centred on the association mean motion. The final list of cluster members contains 129 sources with masses between 0.3 and 0.007 Msolar. We extracted a dozen new low-mass brown dwarfs below 20 MJup, the limit of previous surveys in the region. Finally, we have derived the mass function in Upper Sco over the 0.3-0.01 Msolar mass range, best fit by a single segment with a slope of index α = 0.6 +/- 0.1, in agreement with previous determination in open clusters. Based on observations made with the United Kingdom Infrared Telescope, operated by the Joint Astronomy Centre on behalf of the UK Particle Physics and Astronomy Research Council. E-mail: nl41 [at] star.le.ac.uk (nl41[at]star[dot]le[dot]ac[dot]uk)