According to a report on Digital Spy, show co-creator Johnny Capps told this year's San Diego Comic-Con:
One idea we've been playing around with is to reboot the whole series [for the films]. We would go back in time and start the story again and show different parts that you didn't see in the series. The other idea is to do a continuation, but we haven't decided yet.
The new series, comprising 13 episodes, started filming in March and returns to BBC One later in the year. According to a tweet on the official Merlin Twitter account, Capps also revealed that Anthony Head will guest-star in a reprise of his role as Uther Pendragon in the fifth series. Liam Cunningham and John Bradley have also been cast. In addition, Lindsay Duncan and Janet Montgomery will be returning as Queen Annis and Princess Mithian respectively.
An official YouTube channel for the programme has also been launched, featuring trailers, promos, interviews, and various looks behind the scenes.