Using an improved model code, EUV spectral energy distributions (SEDs) have been computed for a large grid of stellar models spanning the parameter range observed for O and early B stars. These SEDs have been incorporated into an evolutionary population synthesis code to investigate the time-dependence of the integrated SEDs from evolving clusters of massive stars. Purpose of these calculations is to provide a crucial ingredient for the simulations of the photoionized gas in star-forming regions, which then yield information about the star-formation history of observed clusters. The new method used for computing the SEDs renders the influence of spectral lines on the EUV radiation field in identical quality as the high-resolution synthetic spectra used for comparison with observed UV spectra. By means of exemplary UV analyses of individual O stars it has been shown that the models reproduce most features of the observed UV spectra. As the appearance of the observable UV spectrum depends strongly on the spectral shape of the EUV radiation field, this result represents strong evidence that the computed SEDs are on a realistic level, an essential requirement for their application in photoionization calculations. Some minor discrepancies still remain, however, to be resolved in future work. The mass loss rates and terminal velocities from models with consistently calculated hydrodynamics have been shown to reproduce the theoretically predicted wind-momentum--luminosity relation, as well as the predicted metallicity dependence thereof, showing no distinct differences for dwarfs and supergiants. Comparison of the observed UV spectra of a sample of galactic O stars with the synthetic spectra of two sets of models, one based on selfconsistent hydrodynamics, the other on wind parameters derived from an analysis of optical lines, shows discrepancies that are consistent with the scenario of a fragmented stellar wind, although an in-depth investigation of other possible explanations, such as non-solar abundance patterns, remains to be performed. (This will require a detailed spectral analysis and comparative study of a sample of Galactic, LMC, and SMC stars.) The different relations previously obtained for dwarfs and supergiants from analyses of the H-alpha line might therefore be the result of inadequate assumptions made in modelling the optical lines.