Guillermo Del Toro's 'Pacific Rim' will be released in 3-D.

The action film - about a group of humans who use robots to fight off a host of hostile aliens - was originally intended for a release in 2-D, as Guillermo had shot it, but Warner Bros has decided to convert it ahead of its July 2013 release.

The movie stars Charlie Hunnam, Idris Elba and Charlie Day, and Guillermo previously said he was not keen for the film to be 3-D.

He revealed: ''There was a discussion that took a long time to overcome and that was to make the movie 3-D.

''And I didn't want to make the movie 3-D because when you have something big ... to force the 3-D effect on robots and monsters that are supposed to be [big], you are making them miniaturised, you are making them human scale. I knew that 3-D effects sounded like a good idea, but it was going to be counter.''

It is believed one of the reasons is because Warner Bros are keen to release the film in China, which only shows US movies if they are in IMAX or 3-D.