  Since their discovery, submillimetre-selected galaxies (SMGs) have
revolutionized the field of galaxy formation and evolution. From the hundreds
of square degrees mapped at submillimetre wavelengths, only a handful of
sources have been confirmed to lie at z>5 and only two at z>6. All of these
SMGs are rare examples of extreme starburst galaxies with star formation rates
(SFRs) of >1000 M_sun/yr and therefore are not representative of the general
population of dusty star-forming galaxies. Consequently, our understanding of
the nature of these sources, at the earliest epochs, is still incomplete. Here
we report the spectroscopic identification of a gravitationally amplified (mu =
9.3 +/- 1.0) dusty star-forming galaxy at z=6.027. After correcting for
gravitational lensing, we derive an intrinsic less-extreme SFR of 380 +/- 50
M_sun/yr for this source and find that its gas and dust properties are similar
to those measured for local Ultra Luminous Infrared Galaxies (ULIRGs),
extending the local trends to a poorly explored territory in the early
Universe. The star-formation efficiency of this galaxy is similar to those
measured in its local analogues, despite a ~12 Gyr difference in cosmic time.
