This is actually a few weeks old, but as far as I'm aware this is the first time that we have confirmation from someone in the Merlin camp that Season 6 is undergoing definite negotiations behind the scenes. My guess is that they will go the movies route if the cast don't want to sign up for another year.
LET'S JUST HOPE TO BE TRUTH !
http://www.spoilertv.com/2012/11/merlin-bbc-and-shine-want-season-6.html