The pair had a fling in 2003 -- when Britney was at the top of her game and Robin was trying to get recognized. "He was trying hard to develop a reputation as a hot, young singer, and Britney was the biggest thing in show business," the source tells In Touch, sharing that the two met collaborating on a project (which was never released) and "started hooking up."

Robin's reps deny any such relationship. However, the source says that while the "Toxic" singer was newly single, recently splitting from Justin Timberlake, Robin had been dating actress and now wife Paula Patton since high school-- a fact he was keeping quiet then.