The One Direction heartthrob and Delevingne, a frequent running mate of Rihanna's, were seen earlier this month taking in a London performance of Trey Parker and Matt Stone's musical The Book of Mormon.
"They've always agreed to keep things super-casual and private because of their crazy worlds, but Harry genuinely likes Cara a lot -- he has done ever since they met three years ago," the source said. "They've actually had an on/off thing going all this time when they're in the same city, but they've never had the chance to give a real relationship a shot.
"He might profess his love for her -- and he has done that on many occasions -- but she's heard it all before. He said that to her just a month before he went public with Taylor, so I imagine she's learned her lesson."