Since their discovery, submillimetre-selected galaxies(1,2) have revolutionized the field of galaxy formation and evolution. From the hundreds of square degrees mapped at submillimetre wavelengths(3-5), only a handful of sources have been confirmed to lie at z > 5 (refs (6-10)) and only two at z >= 6 (refs (11,12)). All of these submillimetre galaxies are rare examples of extreme starburst galaxies with star formation rates of greater than or similar to 1,000 M-circle dot yr(-1) 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 star formation rate of 380 +/- 50 M-circle dot yr(-1) for this source and find that its gas and dust properties are similar to those measured for local ultra luminous infrared galaxies, 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(13), despite a similar to 12 Gyr difference in cosmic time.