Fermion mixing renormalization and gauge invariance

We study the renormalization of the fermion mixing matrix in the Standard Model and derive the constraints that must be satisfied to respect gauge invariance to all orders. We demonstrate that the prescription based on the on-shell renormalization conditions is not consistent with the Ward-Takahashi Identities and leads to gauge dependent physical amplitudes. A simple scheme is proposed that satisfies all theoretical requirements and is very convenient for practical calculations.