Digital Spy has learned exclusively that the Inspector Lynley Mysteries star will play Agravaine, the uncle of Arthur (Bradley James).
Parker recently appeared in ITV drama Injustice and has also made guest appearances on Lewis and Hotel Babylon.
His film credits include the role of Caspian's father in The Chronicles of Narnia: The Voyage of the Dawn Treader and 2007's St. Trinian's.
The fourth series of Merlin was originally expected to run for ten weeks but it was recently confirmed that the show will return for a full thirteen episodes.
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