[en] Accurate beta spectra measurements are important for radionuclide metrology, validation of theoretical calculations and other applications. To date, most beta spectra were measured with semiconductor detectors and magnetic spectrometers. These methods suffer from low energy resolutions compared to what is achievable with low temperature detectors. Metallic Magnetic Calorimeters (MMCs) with the radionuclide sample embedded in a 4π absorber have proven to be among the best beta spectrometers in terms of energy resolution and threshold, linearity and detection efficiency, notably for low energy beta transitions. In this work, the beta spectrum of ${}^{99}$Tc (Q${}^{-<!--\; ???\; -->}$ = 293.8 keV) that was measured using an optimized MMC detector is presented. It features an energy resolution of approximately 100 eV and two orders of magnitude lower energy thresholds (∼ 0.75 keV) compared to measurements recommended in the literature. The result suggests a spectrum shape which deviates significantly from hitherto theoretical calculations and semi-empirical extrapolations at lower energies (< 25 keV). Furthermore, suboptimal quantum efficiency in microcalorimeters, due to their geometry or energy detection losses caused by high energy beta emitters such as ${}^{36}$Cl (Q${}^{-<!--\; ???\; -->}$ = 709.5 keV), is addressed. In such cases, the resulting beta spectrum distortions can be efficiently corrected with a novel algorithm that relies on extensive Monte Carlo simulations.