The gradual ferromagnetic spin reorientation in hexagonal close packed cobalt (hcp-Co) phase between $230{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $330{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ reported for a Co single crystal [Bertaut et al., Solid State Commun. 1, 81 (1963)] suggests that this phase could not have a hexagonal symmetry. This hypothesis is verified positively by synchrotron radiation diffraction and neutron diffraction on polycrystalline powder cobalt. The analysis of diffraction data has been done by using a specific set of Bragg peaks, which are not sensitive to the stacking faults present in abundance in hcp-Co. The crystal structure of the hcp-type ordered areas of cobalt is described by the monoclinic symmetry with the magnetic space group $C{2}^{\ensuremath{'}}/{m}^{\ensuremath{'}}$. In this monoclinic crystal structure the former hexagonal [001] axis is no longer perpendicular to the hexagonal layers. The hexagonal [001] and [010] axes make an angle equal $\ensuremath{\alpha}\ensuremath{\approx}$ 90.10(1)${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$, while the angle between in-plane [100] and [010] axes equals $\ensuremath{\gamma}\ensuremath{\approx}$ 120.11(1)${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$. The monoclinic symmetry provides an approximate description of the crystal structure of the stacking faulted hcp-Co areas coexisting with fcc-Co areas.