The writer of Doctor Who has revealed how he protested against the “colour blind” casting of a black actor as a Victorian soldier in the BBC show.

Mark Gatiss said he was uneasy about a “brilliant young black actor” being cast as one of the soldiers, because “there weren’t any black soldiers in Victoria’s army”.

He put the decision down to the BBC’s drive to become “more representational and make everything less homogeneously white”.

Gatiss described the “very difficult” email he sent to a colleague explaining his feelings on the matter. It was only after he discovered records showing that there was in fact a single black soldier in Victoria’s army, that he accepted the decision.

The 50-year-old actor and script writer, who is also the co-creator of the BBC drama Sherlock, was speaking to an audience of around 200 Oxford University students about his latest Doctor Who episode, which aired on Saturday evening.

He said he discussed his reservations about the casting with his colleagues, telling them: “This is very difficult email to send, but I don’t think we can do this…these are soldiers from the South African war, they’ve just been fighting the Zulus. There weren’t any black soldiers in Victoria’s army”.

He added: “But obviously we try all the time to be more representational, and to make everything less homogeneously white.

“But then the argument is ‘It’s Doctor Who’, so everything is already a strange and different world where from the time the show came back, Russell T Davies [showrunner of Doctor Who] is very particular about making sure the show had colourblind casting.”

Turning again to his unease at the case of the Victorian army, he told the audience: “But I thought: is this a specific example of where it’s slightly…I didn’t know what the answer was”.

The Doctor Who episode Empress of Mars, which aired on Saturday night, was set in 1888, with the Doctor, Bill and Nardole visiting Mars where there is a war between Victorian soldiers and the Ice Warriors.

Gatiss told how he decided to research the issue, and came across the story of Jimmy Durham, a Sudanese boy who was rescued from the River Nile in 1886 and brought up by soldiers of The Durham Light Infantry regiment.

The soldiers picked the child up and looked after him, giving him the name James Francies Durham, after one of the men who cared for him.

They originally planned to place him in a mission school in Cairo, but decided instead that they would continue to care for him. Jimmy Durham went with the 2nd Battalion to India and Burma and, when he reached the age of 14, he enlisted into The Durham Light Infantry as a bandsman.

“I got kind of obsessed with this great story,” Gatiss said. “This boy, when he was 18 years old, was rescued by the Durham Light Infantry. And they made him their mascot – they called him Jimmy Durham.

“And he became what was called a listed officer, by special dispensation of Queen Victoria. He retired to the North East, married a white girl, and his descendants still live there. It’s an amazing story”.

The black soldier, named “Vincey”, was played by Bayo Gbadamosi who has previously appeared in the BBC television series Casualty and a sci-fi film The Swarm.

Gatiss told the audience: “I thought that was a very interesting dilemma, in terms of there are shows where you can be immediately more diverse and so you should be, and also times when it’s slightly more problematic.”

Asked about the issue of ethnic representation on popular television shows, he said: “To me it’s more about proper sensitivity rather than any kind of – there’s no one standing over you, you just have to try your best at it”.

The BBC’s Director General had pledged that by this year, one in seven of the corporation’s presenters and actors will be black, Asian or minority ethnic.

In 2014, Lord Hall promised 15 per cent of on-air BBC staff will be black, Asian or minority ethnic (BAME) within three years, along with one in ten managers.

Earlier this year the head of Ofcom, which took over the regulation of the corporation from the BBC Trust in April, said that the BBC must stop focusing on middle-aged, middle-class audiences, or risk becoming irrelevant.

Rona Fairhead, the former head of the BBC Trust, warned: “There is so much more to do if we are to reach under-served communities such as 16- 34-year-olds and black, Asian and minority ethnic audiences. For those under-served groups, the bond with the BBC is probably weaker than it was in the past”.

A spokesman for the BBC said: “We cast the best actors for the role”.