  On January 14, 2019, the Major Atmospheric Gamma Imaging Cherenkov telescopes
detected GRB 190114C above 0.2 TeV, recording the most energetic photons ever
observed from a gamma-ray burst. We use this unique observation to probe an
energy dependence of the speed of light in vacuo for photons as predicted by
several quantum gravity models. Based on a set of assumptions on the possible
intrinsic spectral and temporal evolution, we obtain competitive lower limits
on the quadratic leading order of speed of light modification.
