This paper presents a search for new particles in events with one lepton (electron or muon) and missing transverse momentum using 20.3fb⁻¹ of proton-proton collision data at √s = 8TeV recorded by the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. A W′ with Sequential Standard Model couplings is excluded at the 95% confidence level for masses up to 3.24TeV. Excited chiral bosons (W∗) with equivalent coupling strengths are excluded for masses up to 3.21TeV. In the framework of an effective field theory limits are also set on the dark matter-nucleon scattering cross-section as well as the mass scale M∗ of the unknown mediating interaction for dark matter pair production in association with a leptonically decaying W