•The magneto-impedance of a CoP@Cu core–shell microwire was investigated.•The long term stability of the CoP metallic amorphous alloy was tested.•A microstrip waveguide was used for measuring the magneto-impedance.•Giant magneto-impedance effect yielded values as high as 98 % for f = 30 MHz.•The Kittel formula for a plate geometry accounted for FMR in the microwave regime. The giant magneto-impedance effect (GMI) consists in a large variation in the electrical impedance that occurs in some magnetic materials when subjected to a variable external magnetic field. Here we present an experimental study of the GMI in a wide frequency range observed in a structure of the metallic amorphous magnetic alloy CoP electrodeposited on copper microwires by using a microstrip configuration. In the low frequency range, the magnetic field dependence of the impedance is due to the behavior of the circumferential ac susceptibility in the magnetic coating. In the microwave range the behavior of the GMI is dominated by the main dynamical magnetic phenomenon at high frequencies, namely ferromagnetic resonance (FMR). The experimental data are well fit by Kittel equation for the FMR frequency for a ferromagnetic layer with a saturation magnetization similar to the ones reported in previous works. The structural stability of the CoP alloy against aging effects was also investigated. The overall results indicate that the CoP alloy is a good candidate for applications in sensors and in microwave devices.