ITV said the peak number was a combination of people watching on ITV1 and ITV1 +1, some 12 million of whom watched the 8pm ITV1 show.
At its peak, more than half (51%) of those watching television were tuned in to the talent contest, the broadcaster said.
An average audience of 11 million (10.5 million excluding ITV1 +1) watched the new panel of judges, Gary Barlow, Tulisa Contostavlos, Kelly Rowland and Louis Walsh, launch series eight of the show, which was a 47% share of the audience.
A peak audience of 1.6 million (7.2% share) watched The Xtra Factor on ITV2. There was an average audience of 1.5 million across the show.
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