Taken from Bleedingcool.com:
X-Men: Days of Future Past writer and producer Simon Kinberg confirmed that the movie will be part of a shared cinematic universe with the other X-Men movies.
“There’s definitely a sort of overall plan that we’ve all been talking about for the X-Men universe now,” Kinberg explained, “and Deadpool obviously fits into that. So yeah, I guess I would say it’s part of certainly an overall timeline and thought process that goes into these films, some of which is inspired by the comics and some which is inspired by seeing what Marvel’s done with telling a larger tapestry and linking all those movies together, even as they stand independently as well. The same kind of thought is going to go into these X-Men movies at Fox.”