The discovery of quantum oscillations in highly ordered ortho-II phase YBa2Cu3O6+x (YBCO) crystals indicates the existence of a small pocket-like Fermi surface in the normal state of underdoped cuprates which is reached in high magnetic fields. An important task in cuprate research is to identify the charge and/or spin density waves which reconstruct the Fermi surface. The HZB high-field magnet will allow neutron scattering measurements in a field range above the onset field for quantum oscillations in underdoped YBCO, a clean high-temperature superconductor that has been studied extensively by high-field transport and thermodynamic probes. We propose to perform the first scattering experiment in order to characterize the spin- and charge-density-wave correlations in the high-field phase of underdoped YBCO.