Leicester City remain winless away from home in the Premier League this season after being outclassed by Southampton at St Mary's.
Claude Puel's hosts, defeated in each of their last four league games, dominated from the first whistle and deservedly went ahead through James Ward-Prowse's first goal in 22 games.
Ward-Prowse's free-kick helped set up the second for Jay Rodriguez, before Dusan Tadic's late penalty set the seal on a one-sided victory.
Champions Leicester have now gone 11 games without an away league victory this season - having won 11 times on their travels en route to the title.