Context : Aims : We present multiwavelength ( X-ray/optical/near-infrared/millimetre ) observations of GRB 051022 between 2.5 hours and \sim 1.15 yr after the event . It is the most intense gamma-ray burst ( \sim 10 ^ { -4 } erg cm ^ { -2 } ) detected by HETE-2 , with the exception of the nearby GRB 030329 . Methods : Optical and near infrared observations did not detect the afterglow despite a strong afterglow at X-ray wavelengths . Millimetre observations at Plateau de Bure ( PdB ) detected a source and a flare , confirming the association of this event with a moderately bright ( R = 21.5 ) galaxy . Results : Spectroscopic observations of this galaxy show strong [ O II ] , H \beta and [ O III ] emission lines at a redshift of 0.809 . The spectral energy distribution ( SED ) of the galaxy implies A _ { V } ( rest frame ) = 1.0 and a starburst occuring \sim 25 Myr ago , during which the star-forming-rate reached \geq 25 M _ { \odot } /yr . In conjunction with the spatial extent ( \sim 1 ^ { \prime \prime } ) it suggests a very luminous ( M _ { V } = - 21.8 ) blue compact galaxy , for which we also find with Z \sim Z _ { \odot } . The X-ray spectrum shows evidence of considerable absorption by neutral gas with N _ { H,X - ray } = 3.47 ^ { +0.48 } _ { -0.47 } \times 10 ^ { 22 } cm ^ { -2 } ( rest frame ) . Absorption by dust in the host galaxy at z = 0.809 certainly can not account for the non-detection of the optical afterglow , unless the dust-to-gas ratio is quite different than that seen in our Galaxy ( i.e . large dust grains ) . Conclusions : It is likely that the afterglow of the dark GRB 051022 was extinguished along the line of sight by an obscured , dense star forming region in a molecular cloud within the parent host galaxy . This galaxy is different from most GRB hosts being brighter than L ^ { * } by a factor of 3 . We have also derived a SFR \sim 50 M _ { \odot } /yr and predict that this host galaxy will be detected at sub-mm wavelengths .