  Isospin is an almost exact symmetry of strong interactions and the
corrections to the isosymmetric limit are, in general, at the percent level.
For several hadronic quantities relevant for flavour physics phenomenology,
such as pseudoscalar meson masses or the kaon leptonic and semileptonic decay
rates, these effects are of the same order of magnitude of the uncertainties
quoted in current large-scale QCD simulations on the lattice and cannot be
neglected anymore. In this contribution I discuss some recent results for the
leptonic decay rates of light pseudoscalar mesons obtained by the Soton--RM123
Collaboration including the leading-order electromagnetic and strong
isospin-breaking corrections in first principles lattice simulations. The
adopted strategy is within the reach of present lattice technologies and it
allows to determine electromagnetic corrections to physical observables for
which delicate cancellations of infrared divergences occur in the intermediate
steps of the calculation. The application of the developed method to the study
of heavy-light meson and semileptonic decay rates is currently underway.
