The actor made the announcement on Twitter, confirming that he had "just wrapped" filming on the fantasy drama's fourth series.
"Who knew Merlin would be so much fun?" he wrote. "Had a ball. I'm gonna miss racing round the forest."
Daniels recently played James Steel on ITV1's Law & Order: UK and has also appeared in episodes of Spooks and Lark Rise to Candleford.
His film credits include 2005 video game adaptation Doom and upcoming Bryan Singer project Jack the Giant Killer.
Other stars confirmed to appear in future episodes of Merlin include Whitechapel's Phil Davis, former Spooks star Gemma Jones, Battlestar Galactica's James Callis and Rome's Lindsay Duncan.
The show's co-creators Johnny Capps and Julian Murphy recently revealed that a film version of the hit series is also in the works.
Merlin returns to BBC One this Saturday at 7.50pm.
Watch the new Merlin trailer below: