'Avatar 2' won't be released in 2018.

James Cameron has revealed that the hotly anticipated sequel to the 2009 fantasy film has had its release date pushed back yet again, despite plans for it to come out on December 21, 2018.

He told The Toronto Star: ''2018 is not happening.''

James - who is working on four sequels to the movie simultaneously - called it ''an epic undertaking, not unlike building the Three Gorges Dam''.

However, he added: ''We're full-tilt boogie right now.''

James previously spoke about shooting all four movies together, comparing it to a TV miniseries.

He said: ''It's not back-to-back. It's really all one big production. It's more the way you would shoot a miniseries. So, we'll be shooting across all ['Avatar' scripts] simultaneously. So, Monday I might be doing a scene from movie four, and Tuesday I'm doing a scene from movie one.

''We're working across, essentially, eight hours of story. It's going to be a big challenge to keep it all fixed in our minds, exactly where we are, across that story arc at any given point. It's going to be probably the most challenging thing I've ever done.

''I'm sure the actors will be challenged by that as well. It's like, 'No, no, no, no, this person hasn't died yet, so you're still in this phase of your life.' It's a saga. It's like doing all three 'Godfather' films at the same time.''

And he claimed he wasn't worried about when the movies will be released.

He said last year: ''We haven't moved that target yet, but we will if we need to.

''The important thing for me is not when the first one comes out but the cadence of the release pattern.

''I want them to be released as close together as possible, If It's an annual appointment to show up at Christmas, I want to make sure that we're able to fulfill on that promise.''