Robert Pattinson is "really insecure" about his chest.

The 25-year-old 'Twilight' heartthrob reportedly hates his upper half so much that he got deeply "upset" when pictures of him paddle-boarding with his shirt off in Malibu hit the media.

A source said: "He's really insecure about his shirt being off."

Robert is believed to have taken up the sport after being urged by girlfriend Kristen Stewart to lose a few pounds.

A source claimed: "Robert's put on nearly 10lbs eating cheeseburgers and drinking beer while he's not been working.

"Kristen's told him to pull himself together. He hates the gym, so she's encouraged him to take up paddle surfing to try and work off the excess calories."

The 'Cosmopolis' actor - who is also "trying to cut back" on the cigarettes in a bid to improve his health - has previously opened up about his hatred of shirtless scenes, especially when they involve his buff 'Twilight' co-star Taylor Lautner.

He said: "I'd have to make a lifestyle change if I wanted to do shirtless acting. But it seems like every single actor in the world is doing a 'Taylor Lautner' scene - it's crazy.

"They have a scene where they walk out with like a 12-pack. It's like, 'Really? You're playing a normal guy! That doesn't make any sense.' But that's a prerequisite of being an actor. Or you can get fat!"