For its millions of US viewers, the demise of Downton Abbey is a cause for mourning. But is the drama’s huge popularity in America connected to the absence of black characters in its period English setting?

Racism is indeed the explanation, at least as far as Barry Humphries is concerned. The comic actor, famed for his Dame Edna Everage persona, made the suggestion in a magazine interview when he was asked: “Why do you think Downton Abbey is so popular in the States?” The 81 year-old Australian told the Radio Times: “Because there are no black people in it.”

Downton, which began its sixth and final series for American viewers on the PBS network on Sunday, is watched by 26 million in the US.

However, the production company, Carnival Films, have wrestled with Downton’s obvious diversity deficit, a product of the story’s setting in an aristocratic Yorkshire country estate between 1912 and 1925.

The character of Jack Ross, a charismatic black jazz singer and bandleader, played by Gary Carr, was introduced in series four. Writer Julian Fellowes said that Ross, who came to Lady Rose’s rescue on the dancefloor but was the butt of racist remarks from Lady Rosamund, had to be a “positive character”. Fellowes said he would only introduce more black and Asian characters if he could do so in a way that was “historically believable.”

Challenged on the diversity issue, Gareth Neame, executive producer, has said the series had a duty to depict the class system as it was – and “Britain was not a multicultural country in 1920”.

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“The servants would really have had absolutely no exposure to a black person whatsoever,” he has said, adding that making the series more multicultural would have been an exercise in “box-ticking”.

If an absence of black faces helped sell Downton to a mainstream American audience, in thrall to its British comedy of manners, nobody told Michelle Obama. The First Lady begged ITV to send advance DVDs of the series and invited the entire cast for a private tour of the White House in 2012.

Whoopi Goldberg and P Diddy are fans – the rapper starred in a spoof video in which he pretended to be the first black character in his “favourite show of all time” and appeared in a Bafta tribute to the series. The idea that US viewers might be prejudiced against a multicultural British drama is challenged by the success of Luther, the BBC detective series, starring Idris Elba. Broadcast on the BBC America channel, the fourth season opened with a 22 per cent increase in viewers last month. Elba won a Golden Globe and the show has garnered eight Emmy nominations.