IEC/TR 62627-03-03:2013(E) describes the investigation results of high-power reliability for metal-doped optical fibre plug-style attenuators. This report contains the high-power test results for metal-doped optical fibre SC plug-style optical attenuators, the thermal simulation results and the analysis of degradation modes, long-term reliability test results under high-power conditions and the derivation of maximum limit of optical power for guaranteeing long-term operation. Keywords: high-power reliability for metal-doped optical fibre plug-style attenuators