A search for a charged Higgs boson (H +) in $t\bar{t}$ decays is presented, where one of the top quarks decays via t→H + b, followed by H +→ two jets ( $c\bar{s}$ ). The other top quark decays to Wb, where the W boson then decays into a lepton (e/μ) and a neutrino. The data were recorded in pp collisions at $\sqrt{s} = 7~\mathrm {TeV}$ 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 $\mathcal{B}(H^{+} \rightarrow c\bar{s})=100~\%$ .