  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, so far neglected in the literature,
is emphasised. 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 and visible lepton
flavour violating (LFV) effects in the processes tau -> mu ll, tau -> mu rho,
tau -> mu pi and tau -> mu eta^(') are induced.
