ITV rival The Voice pulled ahead with 6million after it was beaten by more than 400,000 viewers when the two shows first went head-to-head in a 25-minute overlap last Saturday.
It comes as the BBC accidentally branded their show as “s***” in a social media blunder.
A source said: “It’s the first time that people at home have been able to choose which show they wanted to watch – and more people have gone with The Voice.
“There was a lot of intrigue to see what Let It Shine was all about and it seems that people are quick to switch over and watch the established programme.
“The ratings battle is bound to intensify when they go on to the live rounds next month.”
On Saturday the official BBC Radio 2 Twitter account embarrassingly shared a post from one angry viewer who was unimpressed by the second episode of Let It Shine.