ABSTRACT We investigate the formation of ultra-diffuse galaxies (UDGs) using the Auriga high-resolution cosmological magnetohydrodynamical simulations of Milky Way–sized galaxies. We identify a sample of 92 UDGs in the simulations that match a wide range of observables such as sizes, central surface brightness, Sérsic indices, colours, spatial distribution, and abundance. Auriga UDGs have dynamical masses similar to normal dwarfs. In the field, the key to their origin is a strong correlation present in low-mass dark matter haloes between galaxy size and halo spin parameter. Field UDGs form in dark matter haloes with larger spins compared to normal dwarfs in the field, in agreement with previous semi-analytical models. Satellite UDGs, on the other hand, have two different origins: ${∼}55{{\ \rm per\ cent}}$ of them formed as field UDGs before they were accreted; the remaining ${∼}45{{\ \rm per\ cent}}$ were normal field dwarfs that subsequently turned into UDGs as a result of tidal interactions.