  We present evidence for a positive angular correlation between bright
submillimetre sources and low-redshift galaxies. The study was conducted using
39 sources selected from 3 contiguous, flux-limited SCUBA surveys,
cross-correlated with optical field galaxies with magnitudes R<23 (with a
median redshift of z~0.5). We find that the angular distribution of submm
sources is skewed towards overdensities in the galaxy population, consistent
with 25+-12 per cent being associated with dense, low-redshift structure. The
signal appears to be dominated by the brightest sources with a flux density
S_850>10mJy. We conduct Monte-Carlo simulations of clustered submm populations,
and find that the probability of obtaining these correlations by chance is less
than 0.4 per cent. The results may suggest that a larger than expected fraction
of submm sources lie at z~0.5. Alternatively, we argue that this signal is most
likely caused by gravitational lensing bias, which may be entirely expected
given the steep submm source counts. Implications for future submm surveys are
discussed.
