  We present photometric redshifts for an uniquely large and deep sample of
522286 objects with i'_{AB}<25 in the Canada-France Legacy Survey ``Deep
Survey'' fields, which cover a total effective area of 3.2 deg^2. We use 3241
spectroscopic redshifts with 0<z<5 from the VIMOS VLT Deep Survey as a
calibration to derive these photometric redshifts. We devise a robust
calibration method which removes systematic trends in the photometric redshifts
and significantly reduces the fraction of catastrophic errors. We use our
unique spectroscopic sample to present a detailed assessment of the robustness
of the photometric redshift sample. For a sample selected at i'_{AB}<24, we
reach a redshift accuracy of \sigma_{\Delta z/(1+z)}=0.037 with \eta=3.7% of
catastrophic error. The reliability of our photometric redshifts is lower for
fainter objects: we find \sigma_{\Delta z/(1+z)}=0.029, 0.043 and \eta=1.7%,
5.4% for samples selected at i'_{AB}=17.5-22.5 and 22.5-24 respectively. We
find that the photometric redshifts of starburst galaxies in our sample are
less reliable: although these galaxies represent only 18% of the spectroscopic
sample they are responsible for 54% of the catastrophic errors. We find an
excellent agreement between the photometric and the VVDS spectroscopic redshift
distributions at i'_{AB}<24. Finally, we compare the redshift distributions of
i' selected galaxies on the four CFHTLS deep fields, showing that cosmic
variance is already present on fields of 0.8 deg^2.
