The Thursday night one-off, airing yesterday evening (June 7) at 9pm, picked up 5.73m (25.5%) and a further 378k (2.1%) on ITV1 +1 an hour later.
Interest in William's life story outmatched that of the Queen, whose ITV programme fetched 5.1m last Friday (June 1).
Over on BBC One, a repeat of hit period drama Call the Midwife could only muster 1.94m (8.6%), but the BBC News still managed to win its slot with 3.2m (16%), beating News at Ten's 2.73m (14.1%) for ITV1 (+1: 225k/2%).
Griff Rhys Jones vehicle Britain's Lost Routes - which faced the might of ITV's soap zone - continued with 2.43m (10.6%) in the 8pm hour.
Big Brother was solid for Channel 5 with 1.44m (8%) at 10pm, prior to which Marco Pierre White's Kitchen Wars launched with 865k (3.9%). Cowboy Builders had 651k (2.8%) at 8pm.
Great British Menu climbed to 2.09m (10%) on BBC Two at 8pm, then Springwatch (2.03m/8.8%) and Springwatch Unsprung (1.62m/7.2%) held steady audiences, before 1.78m (8.5%) caught The London Markets at 9.30pm.
Overall, ITV1 dominated primetime with 23.3% (+1: 0.9%), beating BBC One's 15.2%. BBC Two claimed third place with 7.8%, followed by Channel 4's 4.8% (+1: 0.9%), and Channel 5's 3.8% (+1: 0.2%).
The Big Bang Theory was the most-watched show on the multichannels with 1.22m (5.4%) for E4 at 8pm (+1: 341k/1.5%), while Russell Howard's Good News amused 965k (4.3%) at 9pm and a slot-winning 469k (14.7%) in the graveyard junction of 1am.
Howard's audience at 9pm was over eight times as much as that of ITV2's The Exclusives, which rumbled on with a woeful 118k (0.5%). BBC Three averaged 3.1%, while E4 had 2.4% (+1: 1%).
BARB ratings data supplied by Attentional