A search for a charged Higgs boson (H+) in t (t) over bar decays is presented, where one of the top quarks decays via t -> H(+)b, followed by H+ -> two jets (c (s) over bar). The other top quark decays to Wb, where the W boson then decays into a lepton (e/mu) and a neutrino. The data were recorded in pp collisions at root s = 7 TeV by the ATLAS detector at the LHC in 2011, and correspond to an integrated luminosity of 4.7 fb(-1). With no observation of a signal, 95 \% confidence level (CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons varying between 5 \% and 1 \% for H+ masses between 90 GeV and 150 GeV, assuming B(H+ -> c (s) over bar) = 100 \%.