Lily James is worried 'Mamma Mia! Here We Go Again' is going to ''ruin'' her career.

The 29-year-old actress stars in the upcoming musical sequel as the young Donna Sheridan, who was portrayed by Meryl Streep in the original movie, and she's worried about being compared unfavourably to the screen legend.

She said: ''I've basically set myself up for the biggest fall. This will probably end my career, playing a young Meryl. But I keep having to tell myself, I'm not playing Meryl, I'm playing Donna Sheridan. Everyone who goes to see it will be judging me and I hope I didn't make a silly mistake saying yes. But how could I not say yes?

''This was an opportunity of a lifetime. They were letting me sing ABBA songs, on a boat in Croatia, and dance around. What do you say to that?' No, let me mull it over.' Of course I was on board ... literally.''

Lily spent hours pouring over Meryl's old movies to try and perfect her mannerisms for the role.

She said: ''I watched all the movies she did in her 20s on repeat because that's the same age Donna is when I play her. Trying to study Meryl's movements and her energy, her mannerisms, everything I could - I was writing notes and rehearsing the movements, going over and over.''

But the former 'Downton Abbey' star thinks it was an ''entire waste of time'' because everything the Oscar-winning actress does is instinctive.

She added to heat magazine: ''And it was basically an entire waste of time because you can't learn any of it. It's just her, it's who she is, it's why she's the greatest actor of all time. If it was teachable, what she can do, they'd be churning out Meryl's every week. But she is and always will be one of a kind, never to be repeated again.''