The lead role in Doctor Who has previously been offered to a black actor, showrunner Steven Moffat has revealed.
Moffat, who is to step down from his top role at the BBC drama at the end of next year's series, did not say who the actor was.
He told Doctor Who Magazine: "We've tried. The part has been offered to a black actor. But for various reasons, it didn't work out."
Moffat added: "I certainly don't think there's ever been a problem with making the Doctor black, which is why it should happen one day...
"Should the Doctor be black? Should the Doctor be a woman? So long as the Doctor is still the Doctor, anything is possible."
Before the casting of current Time Lord Peter Capaldi, there had been calls for a black actor to take on the role.
Mixed-race actress Pearl Mackie, whose father is from the West Indies, was cast as the Doctor's companion in April.
Moffat said: "We decided that the new companion was going to be non-white, and that was an absolute decision, because we need to do better on that. We just have to.
"I don't mean that we've done terribly - our guest casts are among the most diverse on television - but I feel as though I could have done better overall."
The casting of Mackie does not rule out casting a non-white actor as the next Doctor, Moffat explained.
He said: "Two non-white leads would be amazing. In fact, a lot of people would barely notice.
"Even a black James Bond would barely raise an eyebrow nowadays. He's still got to be an establishment killer, but a black Bond would be great - and I certainly don't think there's ever been a problem with making the Doctor black, which is why it should happen one day."
Moffat said there was no excuse for non-diverse casting.
"Sometimes the nature of a particular show - historical dramas, for instance - makes diversity more of a challenge, but Doctor Who has absolutely nowhere to hide on this," he said.
"Young people watching have to know that they have a place in the future. That really matters. You have to care profoundly what children's shows in particular say about where you're going to be."
In 2013, Neil Gaiman sparked speculation when he wrote on his blog: "I know one black actor who was already offered the part of the Doctor, and who turned it down.
"You can ask, but seeing that it was something I was told in confidence by the actor in question, you won't get an answer."