  On 5-6 June 2012, Venus will be transiting the Sun for the last time before
2117. This event is an unique opportunity to assess the feasibility of the
atmospheric characterisation of Earth-size exoplanets near the habitable zone
with the transmission spectroscopy technique and provide an invaluable proxy
for the atmosphere of such a planet. In this letter, we provide a theoretical
transmission spectrum of the atmosphere of Venus that could be tested with
spectroscopic observations during the 2012 transit. This is done using
radiative transfer across Venus' atmosphere, with inputs from in-situ missions
such as Venus Express and theoretical models. The transmission spectrum covers
a range of 0.1-5 {\mu}m and probes the limb between 70 and 150 km in altitude.
It is dominated in UV by carbon dioxide absorption producing a broad transit
signal of ~20 ppm as seen from Earth, and from 0.2 to 2.7 {\mu}m by Mie
extinction (~5 ppm at 0.8 {\mu}m) caused by droplets of sulfuric acid composing
an upper haze layer above the main deck of clouds. These features are not
expected for a terrestrial exoplanet and could help discriminating an
Earth-like habitable world from a cytherean planet.
