Bradley James himself has confirmed in an interview with Digital Spy, that the cast was offered a 6th season.
However James had always said he wanted to leave the series on a high and therefor rejected the offer. Where as the producers decided to stick to their 5 year plan instead of continuing the series with a different cast.
In my personal opinion they have made the right decision. The cast of Merlin is a one of the main reasons why Merlin has been such a huge succes. And replacing them just to do one more season seems just wrong.
“We were asked if we wanted to do a series six, but I’d always had in my mind that we’d kind of gone as far as we could – off the back of the five series – and I’ve always harped on about wanting to leave it on a high. I think that’s what’s been achieved this year.”
Source: Cult Box / Digital Spy
Bradley James didn’t rule out that when the script would be good enough, he would however agree to reprise in his role as King or Prince Arthur for a Merlin Movie.
Unfortunately Colin Morgan who plays Merlin, has said that same day in a different interview that in his belief a movie would only dissapoint the fans. And he therefor did not really feel for the idea of making a Merlin movie.
“Now that it’s out in the open that this is the final series, I think fans will be satisfied with how it’s ended. To do any more would possibly go down an avenue that would lead to people being unsatisfied. I think it’s important to end on a high.”
Source: Cult Box/ Digital Spy
So either way IF there will be a continuation of any kind on Merlin, which I don’t think will happen though, it will either mean it will be with a entire different cast or with part of the original cast but without Colin Morgan.