A search for a light charged Higgs boson in t→H^{+}b decays with H^{+} decay to c\bar{b} is presented. The analysis focuses on top quark pair events in which one top quark decays to Wb, with the W boson decaying leptonically, and the other top quark decaying to H^{+}b. The search is based on pp collisions at \sqrt{s}=13 Tev recorded between 2015 and 2018 with the ATLAS detector at the LHC and uses an integrated luminosity of 138.9 fb^{-1}. Data are analyzed in the lepton-plus-jets final state.