The search for Higgs boson production in association with a $W$ or a $Z$ boson, in the decay channel $H \to WW^*$, is performed with a data sample collected with the ATLAS detector at the LHC in proton-proton collisions at centre-of-mass energies \sqrt{s}=7~{\rm TeV} and 8~{\rm TeV}, corresponding to integrated luminosities of 4.5 ${\rm fb}^{-1}$ and 20.3 ${\rm fb}^{-1}$, respectively. The $WH$ production mode is studied in three and two lepton final states, while a four lepton final state is used to search for $ZH$ production. The observed significance is of 2.5 standard deviations while a significance of 0.9 standard deviations is expected for a Standard Model Higgs boson. The ratio of the combined $WH$ and $ZH$ signal yield to the Standard Model expectation, $\mu_{\rm VH}$, is found to be \mu_{\rm VH} =3.0^{+1.3}_{-1.1}{\, {(\rm stat.)}}^{+1.0}_{-0.7}{\,{(\rm sys.)}} for a Higgs boson mass of 125.36 GeV. The $WH$ and $ZH$ channels are also combined with the gluon-gluon fusion and vector boson fusion production modes studied in the $H \to WW^* \to \ell \nu \ell \nu$ decay channel. The results are interpreted in terms of scaling factors of the Higgs boson couplings to vector bosons ($k_V$) and fermions ($k_F$), the combined results are: $|k_V| = 1.06^{+0.10}_{-0.10}$, $|k_F| = 0.85^{+0.26}_{-0.20}$.

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