  We present the discovery of a dusty quasar, SMMJ04135+10277, located behind
the galaxy cluster Abell 478. The quasar was discovered as the brightest source
in a submillimetre survey of high redshift galaxies lensed by foreground rich
clusters of galaxies in a project aimed at studying the cosmic star formation
history of dusty galaxies. With submm fluxes of S_850 = 25+-2.8 mJy and S_450 =
55+-17 mJy this object is one of the brightest submillimetre sources known.
Optical observations revealed a quasar with redshift z=2.837. The quasar was
also detected at shorter infrared wavelengths with the Infrared Space
Observatory. This object is the first quasar discovered by its submm emission.
Given the general lack of overlap between deep submm and X-ray samples, usually
interpreted as a low incidence of active nuclei in submm samples, this is an
unusual object. Analysis of number counts of quasars and of submm galaxies
bears out this suggestion. We compare the properties of SMMJ04135+10277 to
those of optically selected quasars with submm emission, and argue that the
optical faintness results from a large viewing angle with the direction of
relativistic beaming, and not from abnormally high extinction. We also find
indications that the bulk of the submm flux density is not powered by the
quasar nucleus. This conclusion is supported by analysis of the infrared
spectral energy distribution. These results are consistent with previous
observations that quasars at higher redshift tend to have a more prominent cold
dust component, most likely powered by extended star formation in the host
galaxy. The quasar is found to have a total infrared luminosity of
(2.9+-0.5)x10^13 L_sun, dominated by the emission from cool dust. (Abridged)
