A light Higgs boson with substantial branching ratio into invisible channels can occur in a variety of models with: light neutralinos, spontaneously broken lepton number, radiatively generated neutrino masses, additional singlet scalar(s) and/or right handed neutrinos in the extra dimensions of TeV scale gravity. We study the observability of the WH and ZH modes at LHC with H decaying invisibly, by carrying out a detailed simulation with two event generators (HERWIG and PYTHIA) and realistic detector simulations (GETJET and CMSJET). We find that the signal with ‘single lepton plus missing ET’ resulting from WH production suffers from a very large background due to the (off-shell) W∗ production via the Drell–Yan process. In contrast, the ZH mode provides a clean signal in the ‘dilepton plus missing ET’ channel. By exploiting this second signature, we show that invisible branching ratios of Higgs bosons, BRinv, larger than ∼0.42 (0.70) can be probed at 5σ level for MH=120 (160) GeV, respectively, assuming an accumulated luminosity of L=100 fb−1.