A search for the Higgs boson has been performed in the $H\ensuremath{\rightarrow}W{W}^{(*)}\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}\ensuremath{\nu}{\ensuremath{\ell}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ channel ($\ensuremath{\ell}=e/\ensuremath{\mu}$) with an integrated luminosity of $2.05\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range $110\text{ }\text{ }\mathrm{GeV}<{m}_{\mathrm{H}}<300\text{ }\text{ }\mathrm{GeV}$. The observations exclude the presence of a standard model Higgs boson with a mass $145<{m}_{\mathrm{H}}<206\text{ }\text{ }\mathrm{GeV}$ at 95% confidence level.