There's been a significant shake-up in the world of mutants.

"X-Men: First Class" writer-director Matthew Vaughn has reportedly left the upcoming sequel, "X-Men: Days of Future Past," just weeks before filming was scheduled to begin. 

The film's producer Bryan Singer, who directed the first two "X-Men" films (Brett Ratner directed "X-Men 3: The Last Stand") and acted as a consultant on "First Class" is rumored to be returning to the director's chair for "Future Past," according to Deadline.

While some of the continuity of the 1960s-set "First Class" seemed to clash with that of the first three "X-Men" films, adapting the celebrated "Days of Future Past" storyline offers the studio a chance to mix events and characters from both timelines. It therefore could work for Singer to step back into the director's chair, however Vaughn's departure is still a huge surprise.

"X-Men" star Patrick Stewart previously hinted that he'll reprise his role as Professor X in the film, possibly alongside James McAvoy as his younger self. 

The sequel is scheduled to start shooting in January, allowing co-star Jennifer Lawrence (Mystique) to wrap filing on "The Hunger Games: Catching Fire." James McAvoy, Michael Fassbender and the other young mutants will also be returning. It's tentatively scheduled to be released July 18, 2014.

After directing 2000's "X-Men" and 2003's "X2: X-Men United," Singer stepped away from the franchise in order to direct "Superman Returns" and "Valkyrie." His latest film, "Jack the Giant Killer," will hit screens March 1, 2013.

Vaughn meanwhile has plans for another film for Fox, likely the drama "Secret Service," based on the comic book series that he and Mark Millar ("Kick-Ass") have been working on together. Vaughn directed the film version of "Kick-Ass," but is only acting as a producer on the sequel, which is currently in production and will be released June 28, 2013.

Who do you think should direct "X-Men: Days of Future Past"? Should Bryan Singer return?