  We report on deep near-infrared observations obtained with the Wide Field
Camera 3 (WFC3) onboard the Hubble Space Telescope (HST) of the first five
confirmed gravitational lensing events discovered by the Herschel Astrophysical
Terahertz Large Area Survey (H-ATLAS). We succeed in disentangling the
background galaxy from the lens to gain separate photometry of the two
components. The HST data allow us to significantly improve on previous
constraints of the mass in stars of the lensed galaxy and to perform accurate
lens modelling of these systems, as described in the accompanying paper by Dye
et al. We fit the spectral energy distributions of the background sources from
near-IR to millimetre wavelengths and use the magnification factors estimated
by Dye et al. to derive the intrinsic properties of the lensed galaxies. We
find these galaxies to have star-formation rates of approximately 400 to 2000
M_sol/yr, with approximately (6-25)x10^10 M_sol of their baryonic mass already
turned into stars. At these rates of star formation, all remaining molecular
gas will be exhausted in less than 100 Myr, reaching a final mass in stars of a
few 10^11 M_sol. These galaxies are thus proto-ellipticals caught during their
major episode of star formation, and observed at the peak epoch z=1.5-3 of the
cosmic star formation history of the Universe.
