  The first multi-messenger detection of a binary neutron star merger,
GW170817, brought to the forefront the structured jet model as a way to explain
multi-wavelength observations taken more than a year after the event. Here we
show that the high-latitude emission from a structured jet can naturally
produce an X-ray plateau in gamma-ray burst (GRB) light curves, independent of
the radiation from an external shock. We calculate the radiation from a
switched-off shell featuring an angular structure in both its relativistic bulk
motion and intrinsic brightness. Our model is able to explain the shallow decay
phase (plateau) often observed in GRB X-ray light curves. We discuss the
possible contribution of the structured jet high-latitude emission to other
distinctive features of GRB X-ray light curves, and its capability to explain
the chromatic optical/X-ray light curve properties.
