  We present an analysis of the morphology and profiles of the dust continuum
emission in 153 bright sub-millimetre galaxies (SMGs) detected with ALMA at S/N
ratios of $>8$ in high-resolution $0.18''$ ($\sim1$kpc) 870$\mu$m maps. We
measure sizes, shapes and light profiles for the rest-frame far-infrared
emission from these luminous star-forming systems and derive a median effective
radius ($R_e$) of $0.10''\pm0.04''$ for our sample with a median flux of
$S_{870}=5.6\pm0.2$mJy. We find that the apparent axial ratio ($b/a$)
distribution of the SMGs peaks at $b/a\sim0.63\pm0.24$ and is best described by
triaxial morphologies, while their emission profiles are best fit by a Sersic
model with $n\simeq1.0\pm0.1$, similar to exponential discs. This combination
of triaxiality and $n\sim1$ Sersic index are characteristic of bars and we
suggest that the bulk of the 870$\mu$m dust continuum emission in the central
$\sim2$kpc of these galaxies arises from bar-like structures. By stacking our
870$\mu$m maps we recover faint extended dust continuum emission on $\sim4$kpc
scales which contributes $13\pm1$% of the total 870$\mu$m emission. The scale
of this extended emission is similar to that seen for the molecular gas and
rest-frame optical light in these systems, suggesting that it represents an
extended dust and gas disc at radii larger than the more active bar component.
Including this component in our estimated size of the sources we derive a
typical effective radius of $\simeq0.15''\pm0.05''$ or $1.2\pm0.4$kpc. Our
results suggest that kpc-scale bars are ubiquitous features of high
star-formation rate systems at $z\gg1$, while these systems also contain
fainter and more extended gas and stellar envelopes. We suggest that these
features, seen some $10-12$Gyrs ago, represent the formation phase of the
earliest galactic-scale components: stellar bulges.
