A search is presented for production of a heavy up-type quark (t′) together with its antiparticle, assuming a significant branching ratio for subsequent decay into a W boson and a b quark. The search is based on 4.7 fb−1 of pp collisions at √s=7 TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the lepton + jets final state, characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least three jets. The analysis strategy relies on the substantial boost of the W bosons in t′t′ signal when mt′≳400 GeV. No significant excess of events above the Standard Model expectation is observed and the result of the search is interpreted in the context of fourth-generation and vector-like quark models. Under the assumption of a branching ratio BR(t′→Wb)=1BR(t′→Wb)=1, a fourth-generation t′t′ quark with mass lower than 656 GeV656 GeV is excluded at 95% confidence level. In addition, in light of the recent discovery of a new boson of mass ∼126 GeV∼126 GeV at the LHC, upper limits are derived in the two-dimensional plane of BR(t′→Wb)BR(t′→Wb) versus BR(t′→Ht)BR(t′→Ht), where H is the Standard Model Higgs boson, for vector-like quarks of various masses.