When sufficiently close to the Sun, ices in cometary nuclei sublimate, ejecting in space dust and gases whose compositions can be derived by the remote spectral analysis of the cometary atmospheres. Those very rich spectra reveal a host of constituents from simple radicals like OH and CN in the optical range, to relatively complex organic molecules in the infrared and sub-millimeter domain. The majority of these molecules are made of C, H, O and N atoms. Iron, nickel and a few other siderophile atoms have only been detected in two exceptional sungrazer comets in a century and a half. Here we report that free atoms of iron and nickel are ubiquitous in cometary atmospheres as revealed by high-resolution spectra obtained in the near-ultraviolet with the ESO Very Large Telescope for a large sample of comets of various dynamical origins. The emissions of NiI and FeI in cometary comae have been overlooked until now and, surprisingly, are even detected at large heliocentric distances. The abundances of both species appear to be of the same order of magnitude, contrasting with the typical solar system abundance and providing clues about their origins in comet nuclei.