Context. Two scenarios have been proposed to match the existing observational constraints of the object HR 6819. The system could consist of a close inner B-type giant + black hole (BH) binary with an additional Be companion on a wide orbit. Alternatively, it could be a binary composed of a stripped B star and a Be star in a close orbit. Both scenarios make HR 6819 a cornerstone object, either as the closest stellar BH to Earth, or as an example of an important transitional, non-equilibrium phase for Be stars with solid evidence to back its nature. Aims. We aim to distinguish between the two scenarios for HR 6819. Both models predict two luminous stars but with very different angular separations and orbital motions. Therefore, the presence of bright sources in the 1-100 milli-arcsec (mas) regime is a key diagnostic to determine the nature of the HR 6819 system. Methods. We obtained new high-angular resolution data with VLT/MUSE and VLTI/GRAVITY of HR 6819. The MUSE data are sensitive to bright companions at large scales, whilst the interferometric GRAVITY data are sensitive down to separations of order mas scales and large magnitude differences. Results. The MUSE observations reveal no bright companion at large separations and the GRAVITY observations indicate the presence of a stellar companion at an angular separation of ∼ 1.2 mas, moving on the plane of the sky over a time scale compatible with the known spectroscopic 40 day period. Conclusions. We conclude that HR 6819 is a binary system and that no BH is present in the system. The unique nature of HR 6819, and its proximity to Earth make it an ideal system to quantitatively characterise the immediate outcome of binary interaction and probe how Be stars form.