The 24-year-old actress has previously stated she was looking forward to the movie - entitled Wrath of the Titans and starring Liam Neeson and Sam Worthington - but will no longer be involved.
She said: "I'm not doing the sequel."
When pressed on why she has pulled out, she replied: "I can't go into it. I'm just not."
There has been no public announcement made from the creators of the film about why the actress has decided not to go ahead with the sequel.
British-born Gemma also admits she still "craves" a certain film experience which she is yet to have.
The Tamara Drewe star added: "You have to embrace the epic to do those sorts of films. If, like me, you crave an intimate experience, you're not necessarily going to get it with that kind of movie. I still haven't had the film experience that I crave."
Discussing the movie previously, Gemma confessed she was looking forward to working on it.
She said: "It's always interesting, when you've done something, to revisit it 'cause you have a different take on it.
"You remember the experience and can develop from there. It will be interesting. I really liked working with the actors."
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