Sue Moretta was performing the Barbra Streisand classic Don’t Rain On My Parade.
But the camp judge got so carried away with the gay anthem that he jumped up and whipped off his slacks.
“Have I had nightmares? A little bit”
David, 45, then proceeded to shake his bottom at Sue – and his front at Simon Cowell.
Millions will see the shock audition tonight.
Sue, 58, joked last night: “Have I had nightmares? A little bit.
“David was wearing pants but he had his back to me and was facing Simon. I don’t know what it looked like from the other side but Simon didn’t know where to look.
“He had his hands over his eyes.” Sue, from Clayton-le-Moors, Lancs, managed to finish her song without missing a note.
She explained: “I didn’t get distracted. I’m a pro.”
Former BGT finalist Tina Humphrey, who performed with dancing dog Chandi in 2012, has died of cancer aged 45.