  We used 40 high resolution, high S/N QSO spectra at 2.1<z<4.7 to search for
the signature of the proximity effect in the HI Lyalpha forest. Comparing the
effective optical depth near each QSO with the expected one, we clearly detect
the proximity effect on the combined QSO sample and towards each individual
QSO. The observed proximity effect strength distribution (PESD) is asymmetric
towards a weak effect. We demonstrate that this is not simply an effect of
gravitational clustering around QSOs. Comparing simulated PESDs with
observations, we argue that the averaging method to determine the UVB intensity
J is heavily biased towards high values because of the PESD asymmetry. Using
instead the mode of the PESD provides an unbiased estimate of J. For our sample
its modal value is log(J)=-21.51+/-0.15 (in units of ergcm^-2s^-1Hz^-1sr^-1) at
z=2.73. We estimated the excess HI absorption attributed to gravitational
clustering. On scales of ~3 Mpc, only a minority of QSOs shows overdensities of
up to a factor of a few in tau_eff; these are exactly the objects with the
weakest proximity effects. After removing them, we redetermined the UVB
intensity arriving at log(J)=-21.46+0.14-0.21. This is the most accurate
measurement of J to date. We present a new diagnostic based on the shape of the
PESD which strongly supports our conclusion that there is no systematic
overdensity bias for the proximity effect. This additional diagnostic breaks
the otherwise unavoidable degeneracy of the proximity effect between UVB and
overdensity. We estimated the redshift evolution of J and found tentative
evidence for a mild decrease with increasing redshift. Our results are in
excellent agreement with predictions for the evolving UVB intensity, supporting
the notion of a substantial contribution of star-forming galaxies.

Astrophysics