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We present the final results from our ultra-deep spectroscopic campaign withFORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidatesselected from our VLT/Hawk-I imaging survey over three independent fields. Inparticular we report on two newly discovered galaxies at redshift ~6.7 in theNTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWsof the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also presentthe results of ultra-deep observations of a sample of i-dropout galaxies, fromwhich we set a solid upper limit on the fraction of interlopers. Out of the 20z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This issystematically below the expectations drawn on the basis of lower redshiftobservations: in particular there is a significant lack of objects withintermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend forthe fraction of Ly-alpha emission in LBGs that is constantly increasing fromz~3 to z~6 is most probably reversed from z~6 to z~7.  Explaining the observed rapid change in the LAE fraction among the drop-outpopulation with reionization requires a fast evolution of the neutral fractionof hydrogen in the Universe. Assuming that the Universe is completely ionizedat z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), wefind that our data require a change of the neutral hydrogen fraction of theorder Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escapefraction does not increase dramatically over the same redshift interval.

Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization