  The steep source counts and negative K-corrections of bright submillimetre
galaxies (SMGs) suggest that a significant fraction of those observed at high
flux densities may be gravitationally lensed, and that the lensing objects may
often lie at redshifts above 1, where clusters of galaxies are difficult to
detect through other means. In this case follow-up of bright SMGs may be used
to identify dense structures along the line of sight. Here we investigate the
probability for SMGs to experience strong lensing, using the latest N-body
simulations and observed source flux and redshift distributions. We find that
almost all high redshift sources with a flux density above 100mJy will be
lensed, if they are not relatively local galaxies. We also give estimates of
the fraction of sources experiencing strong lensing as a function of observed
flux density. This has implications for planning follow-up observations for
bright SMGs discovered in future surveys with SCUBA-2 and other instruments.
The largest uncertainty in these calculations is the maximum allowed lensing
amplification, which is dominated by the presently unknown spatial extent of
SMGs.
