puts stringent bounds on the parameters of many extensions of the SM such as Supersymmetry (SUSY) [@Yuval]. So far, FCNC processes have been explored mainly in particle–antiparticle mixing and in “semihadronic” weak decays, which permit a clean theoretical description. In the kaon system, examples of the latter type are the decays $K\to\pi\,l^+l^-$ and $K\to\pi\,\nu\bar\nu$. In the $B$ system, the decays that have received the most attention are $B\to X_s\,\gamma$, $B\to X_s\,l^+l^-$ and $B\to X_s\,\nu\bar\nu$, where $X_s$ can be any final state, exclusive or inclusive, containing a strange quark [@BaBar].

In the present paper, we explore in detail how New Physics could affect purely hadronic decays such as $B\to\pi K$, which are sensitive to isospin- or, more generally, SU(3) flavor-violating interactions. In the SM, the main contributions to the decay amplitudes for these processes come from the penguin-induced FCNC transition $\bar b\to\bar s q\bar q$, which by far exceeds a small, Cabibbo-suppressed $\bar b\to\bar u u\bar s$ contribution from $W$-boson exchange. Because of a fortunate interplay of isospin, Fierz and flavor symmetries, the theoretical description of the charged decays $B^\pm\to\pi K$ in the SM is clean despite the fact that these are exclusive nonleptonic decays [@NR; @me]. Isospin violation arises through 