A search for a Higgs boson has been performed in the $H\ensuremath{\rightarrow}WW\ensuremath{\rightarrow}\ensuremath{\ell}\ensuremath{\nu}jj$ channel in $1.04\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of $pp$ collision data at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$ recorded with the ATLAS detector at the Large Hadron Collider. No significant excess of events is observed over the expected background and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range $240\text{ }\text{ }\mathrm{GeV}<{m}_{H}<600\text{ }\text{ }\mathrm{GeV}$. The best sensitivity is reached for ${m}_{H}=400\text{ }\text{ }\mathrm{GeV}$, where the 95% confidence level upper bound on the cross section for $H\ensuremath{\rightarrow}WW$ production is 3.1 pb, or 2.7 times the standard model prediction.