The film, a follow-up to the 2011 prequel X-Men: First Class, will feature Michael Fassbender and James McEvoy in the roles of Magneto and Professor X respectively.
However, director Bryan Singer, who makes a return to the franchise after directing the first two entries in the series, has announced that McKellan and Stewart will also be making a return.
He tweeted: "Thrilled to announce @ianmckellen118 & @SirPatStew are joining the cast of #XMEN #DaysOfFuturePast #magneto #professorX More to come.."
The two English actors played the roles of Magneto and Professor X in the original trilogy, only to be replaced by Fassbender and McEvoy as the younger iterations of the characters.
Stewart had previously hinted he may be reprising his role for the new movie at Montreal Comic-Con earlier in the year.
Days of Future Past is to be based on Chris Claremont's Uncanny X-Men arc, the storyline that introduced Cyclops and Phoenix's alternate reality daughter Rachel Summers/Grey to the X-Men universe.
Watch Michael Fassbender discuss Days of Future Past below: