VGFacts

Ueda's main inspiration for Ico was Another World, which used cinematic cutscenes and lacked any head-up display elements as to play like a movie. It also featured an emotional connection between two characters, despite the use of minimal dialog. Ueda also cited Lemmings, Flashback and the original Prince of Persia games as influences.

Sources:
http://www.famitsu.com/game/news/2007/04...5,0,0.html
http://www.gamespot.com/news/6164199.html
http://web.archive.org/web/2008120614392...ng-part-2/