The processes of deep inelastic electron - proton (ep) scattering by charged current are suggested for investigation of the proton spin. An approach is proposed for the determination of the contributions of individual quark flavours and valence quarks to the proton spin. The electroweak radiative corrections of O to the observable quantities are calculated. The numerical calculations of the cross sections and the longitudinal polarization asymmetries taking into consideration electroweak corrections at HERA energies have been made.