  Bright galaxies at sub-millimetre wavelengths from Herschel are now well
known to be predominantly strongly gravitationally lensed. The same models that
successfully predicted this strongly lensed population also predict about one
percent of faint $450{\mu}$m-selected galaxies from deep James Clerk Maxwell
Telescope (JCMT) surveys will also be strongly lensed. Follow-up ALMA campaigns
have so far found one potential lens candidate, but without clear compelling
evidence e.g. from lensing arcs. Here we report the discovery of a compelling
gravitational lens system confirming the lensing population predictions, with a
$z_{s} = 3.4 {\pm} 0.4$ submm source lensed by a $z_{spec} = 0.360$ foreground
galaxy within the COSMOS field, identified through public JWST imaging of a
$450{\mu}$m source in the SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES)
catalogue. These systems will typically be well within the detectable range of
future wide-field surveys such as Euclid and Roman, and since sub-millimetre
galaxies are predominantly very red at optical/near-infrared wavelengths, they
will tend to appear in near-infrared channels only. Extrapolating to the
Euclid-Wide survey, we predict tens of thousands of strongly lensed
near-infrared galaxies. This will be transformative for the study of dusty
star-forming galaxies at cosmic noon, but will be a contaminant population in
searches for strongly lensed ultra-high-redshift galaxies in Euclid and Roman.
