Panettiere reportedly ripped off a Velcro-lined version of her cheerleader outfit to reveal red lingerie while serenading Ventimiglia, who had turned 31.
A source is quoted as saying: "Hayden gave Milo an unforgettable birthday surprise by morphing from her bouncy cheerleader character in Heroes to a bump-and-grind striptease.
"She asked the wardrobe girls to make her a Velcro-lined version of the outfit she wears on the show. As she sang 'Happy Birthday' to Milo in front of cast and crew during lunch she ripped off the costume to show off her sexy red lingerie. Milo's face went bright red."