Lepton flavour universality (LFU) in B-decays is revisited by considering a class of semileptonic operators defined at a scale
Lambda above the electroweak scale v. The importance of quantum effects is emphasised ref.. We construct the low-energy effective Lagrangian taking into account the running effects from Lambda down to v through the one-loop renormalization group equations (RGE) in the limit of exact electroweak symmetry and QED RGEs from v down to the 1 GeV scale. The most important quantum effects turn out to be the modification of the leptonic couplings of the vector boson Z and the generation of a purely leptonic effective Lagrangian. Large LFU breaking effects in Z and tau decays as well as visible lepton flavour violating (LFV)
effects in tau decays are induced.