The Game of Thrones star will appear in the fantasy drama's fifth series, Digital Spy can exclusively reveal.
"We can confirm that... Liam Cunningham is going to appear in series five, along with returning guest stars Lindsay Duncan (Queen Annis) and Janet Montgomery (Princess Mithian)," the BBC said.
Digital Spy also has an exclusive first-look image of Cunningham on-set (above), as well as a behind-the-scenes picture of Merlin star Colin Morgan (below).
Merlin creators Johnny Capps and Julian Murphy previously told DS that the series could continue beyond the upcoming fifth run.
"We've always planned five series, but it depends on how popular the show is," said the pair. "There's every possibility it could continue."
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Watch a clip from the last series of Merlin below: