Unfortunately at 26th of November 2012, BBC announced that Merlin Season 5 will be the final season. There will be no further movies or spin offs of the TV series with the known cast. Shine and BBC have let us known they had wanted to continue, but neither the cast nor the producers were up for another season. They decided to stick to their 5 year plan and finish the series with a final two episodes airing on 22nd and 24th of December 2012.
** Update end **
According to Claude Albouze, Production Director of Merlin in France: “There will certainly be a S6 of Merlin, it’s just a matter of negotiations because the first contract was signed for a period of five years and has to be renewed, he assures that both BBC and Shine absolutely want a S6 of Merlin, they want it desperately, they both want to continue the adventure”. They are just like us guys, they want a S6!
Source: French TV, Channel 3 news http://www.youtube.com/watch?v=03H_nk1ijvA&feature=share thanks to BJsRealm