In several extensions of the Standard Model, the top quark can decay into a bottom quark and a light charged Higgs boson H +, t → bH +, in addition to the Standard Model decay t → bW. Since W bosons decay to the three lepton generations equally, while H + may predominantly decay into τν, charged Higgs bosons can be searched for using the violation of lepton universality in top quark decays. The analysis in this paper is based on 4.6 fb−1 of proton-proton collision data at s√=7 TeV collected by the ATLAS experiment at the Large Hadron Collider. Signatures containing leptons (e or μ) and/or a hadronically decaying τ (τ had) are used. Event yield ratios between e + τ had and e + μ, as well as between μ + τ had andμ + e, final states are measured in the data and compared to predictions from simulations. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviation from the Standard Model predictions is observed. With the assumption that the branching fraction B (H + → τν) is 100%, upper limits in the range 3.2%–4.4% can be placed on the branching fraction B (t → bH +) for charged Higgs boson masses m H+ in the range 90–140 GeV. After combination with results from a search for charged Higgs bosons in tt¯ decays using the τ had + jets final state, upper limits on B (t → bH +) can be set in the range 0.8%–3.4%, for m H+ in the range 90-160 GeV.