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 ≳1,000 M⊙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 (μ = 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⊙ 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 ∼12 Gyr difference in cosmic time.