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 \simeq 0.5 ) .
We find that the angular distribution of submm sources is skewed towards overdensities in the galaxy population , consistent with 25 \pm 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 \mu { m } } > 10 mJy .
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 \simeq 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 .