The Dark Knight star was cast as the legendary car manufacturer last year (15), but the part required Christian to pack on the pounds to accurately portray the late Italian racing driver.

The British actor has previously tested his health for roles in the past, losing and/or gaining weight for The Fighter, The MAChinist and American Hustle, but this time, he decided it was too much and walked away from the project in January (16).

Now Christian has opened up about his concerns, admitting his advancing age means his body can no longer tolerate drastic changes to his waistline.

"I'm just getting older," the 42-year-old told The Associated Press. "I could lose and gain so much weight before and didn't feel any different. I just felt invincible."

It's not just the weight gain which worried the father-of-two: "I've had a series of accidents - motorcycle accidents, concussions," he explained. "I've got so much metal in my body holding me together.

"It's a great shame because Michael is a very good friend and it was a very special thing, but I just had too many red flags. And I'm a dad. I'm not f**king around with that. So I just said I've got to surrender on this one."

Director Mann has yet to announce a replacement for the biopic.

Meanwhile, it wasn't the only movie Christian wanted to drop from his work schedule; he has also confessed to almost walking off the Las Vegas set of Terrence Malick's new romantic drama, Knight of Cups.

The filmmaker, who also directed the actor in 2005's The New World, is known for his unconventional approach to moviemaking, but his penchant for developing the script as the cameras rolled nearly caused Christian to quit as he was simply exhausted after a long night shoot.

"(For Knight of Cups) there was a morning we were in Vegas and we'd been walking all night through the neon (lights of The Strip) and everything and through the casinos," he began, "and the next morning we were all a bit bleary-eyed and Terry was just kind of chucking a few lines to me and I was saying them and he was like, 'Just riff, just do what you want,' which was rare because most of the time he just wanted me to listen...

"So this one morning, I went, 'That's it. I'm done. I've got no interest. I don't really care. I don't want to make this anymore. I'm done. I've got nothing.' And he went, 'Start filming immediately, this is fantastic.' Because it's sincere. I'm always an actor who wants to quit acting and he just went, 'Great'."

And Christian admits he battles feelings of wanting to drop out of projects all the time: "I do that every project. I do! I know I'm going to get over it. I'm always doing it. My entire life... There have been hundreds of times when, no doubt about it, I'm done. But then something will happen and I'll go, 'B**ody hell, I'm an addict!'"