Doctor Who star Matt Smith is to leave his role as the Doctor at the end of this year, the BBC has announced.
After four years as the Time Lord on the BBC One show, viewers will see Smith's Doctor regenerate in the 2013 Christmas special.
The 30-year-old actor said working on the show had been "the most brilliant experience".
Doctor Who marks its 50th anniversary in November with a special episode, which Smith has already filmed.
The BBC said Smith's "spectacular exit" was yet to be revealed and would be "kept tightly under wraps".
Smith first stepped into the Tardis as the 11th Doctor in 2010. Taking over from David Tennant, he was the youngest actor to play the role.
Speaking after the announcement, he said he was "incredibly proud" of what the show had achieved over the last four years under Steven Moffat, the show's lead writer and executive producer.
Smith also thanked fans around the world for their "truly remarkable" dedication to the show.
His Doctor has shared his adventures with Amy Pond (Karen Gillan), Rory Williams (Arthur Darvill) and most recently, Clara Oswald (Jenna-Louise Coleman).
Referring to his time-travelling companions, Smith said: "It's been an honour to play this part, to follow the legacy of brilliant actors, and helm the Tardis for a spell with 'the ginger, the nose and the impossible one'. But when ya gotta go, ya gotta go..."
Show runner Steven Moffat said: "Every day, on every episode, in every set of rushes, Matt Smith surprised me. The way he'd turn a line, or spin on his heels, or make something funny, or out of nowhere make me cry - I just never knew what was coming next.
"The Doctor can be clown and hero - often at the same time - and Matt rose to both challenges magnificently. And even better than that, given the pressures of this extraordinary show, he is one of the nicest and hardest-working people I have ever had the privilege of knowing. Whatever we threw at him - sometimes literally - his behaviour was always worthy of the Doctor."
The announcement of Smith's exit is likely to spark months of fevered speculation about his replacement.
'Still so exciting' "Somewhere out there right now - all unknowing, just going about their business - is someone who's about to become the Doctor," Moffat hinted.
"A life is going to change, and Doctor Who will be born all over again. After 50 years, that's still so exciting."
"Doctor Who has been the most brilliant experience for me as an actor and a bloke, and that largely is down to the cast, crew and fans of the show.
"I'm incredibly grateful to all the cast and crew who work tirelessly every day to realise all the elements of the show and deliver Doctor Who to the audience. Many of them have become good friends and I'm incredibly proud of what we have achieved over the last four years.
"Having Steven Moffat as show runner write such varied, funny, mind bending and brilliant scripts has been one of the greatest and most rewarding challenges of my career. It's been a privilege and a treat to work with Steven - he's a good friend and will continue to shape a brilliant world for the Doctor.
The fans of Doctor Who around the world are unlike any other; they dress up, shout louder, know more about the history of the show (and speculate more about the future of the show) in a way that I've never seen before.
"Your dedication is truly remarkable. Thank you so very much for supporting my incarnation of the Time Lord, number 11, who I might add is not done yet - I'm back for the 50th anniversary and the Christmas special.
"It's been an honour to play this part, to follow the legacy of brilliant actors, and helm the Tardis for a spell with 'the ginger, the nose and the impossible one'. But when ya gotta go, ya gotta go and Trenzalore calls."
Thank you guys.