Actor Matt Smith was 11th actor to portray Doctor Who. (Canadian Press)

The BBC says Matt Smith is stepping down from the lead role in long-running sci-fi series Doctor Who, spurring intense speculation about his replacement.

The broadcaster said Sunday that Smith will leave after a November episode to mark the show's 50th anniversary, and a Christmas special.

The titular Doctor is a time-travelling extra-terrestrial Time Lord who can regenerate into new bodies. Smith was the latest of a dozen actors who have played the character since the series began in 1963. When he was unveiled in 2009 as the newest Doctor, he also became the youngest one at age 26.

Smith, who took over the role from David Tennant in 2010, said playing the Doctor had been "the most brilliant experience for me as an actor and a bloke."

Doctor Who is one of the BBC's most popular programs, and Smith's tenure has seen the show gain new fans in the United States, where it is shown on BBC America.

The BBC did not announce Smith's replacement, but fans took to the internet to speculate about casting of the 12th Doctor.

Bookmaker William Hill made Being Human star Russell Tovey and Harry Potter alumnus Rupert Grint 10-1 favorites, followed by stage star Rory Kinnear and Homeland actor David Harewood.

William Hill also offered 8-1 odds on the next Doctor being female.