This might be the 21st century but it is still notably rare for a British TV drama or comedy to centre on a gay couple. Aside from Queer as Folk and Sugar Rush - both made for Channel 4 - few fictional television programmes have focused centrally on lesbian, gay, bisexual or transgender (LGBT) relationships.

But new ITV sitcom Vicious, starting tonight at 9pm, could change things. Starring theatrical luminaries Sir Ian McKellen and Sir Derek Jacobi, the comedy is about Freddie and Stuart, an ageing gay couple who have lived together in their small London flat for nearly 50 years. They enjoy reading, walking their dog and bickering.

Written and created by Will & Grace writer Gary Janetti and award-winning playwright Mark Ravenhill, Vicious is the first time that an LGBT couple has been cast at the fore of an ITV comedy. But what makes the show even more unusual is its incidental portrayal of the characters’ sexuality. They are not presented as funny because they are gay (although the show is very funny for other reasons), and their sexuality is not a dramatic vehicle in itself.

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Sir McKellen says Stuart and Freddie’s relationship is something of an advent for television. “It’s not aiming to shock people. It won’t alarm anyone. It isn’t a satire or an exposé of gay life. These characters just happen to be gay. For me it is as if TV has grown-up,” he says.

“In the past gay characters in sitcoms have been figures of fun. They were funny because they were gay. But I like the fact that these characters are funny because of the people they are. That’s a real advance.”

A report by the BBC last year found that examples of LGBT characters in comedy scenarios were often too reliant on outdated stereotypes. The report found that gay male representation is improving on TV, but overtly camp gay men were still depicted as the norm, especially in comedy scenarios.

Doctor Who was commended for including gay character Captain Jack, whose sexuality is incidental to the plot and not the driving-force behind his actions, but the report highlighted that this was a rare portrayal of an LGBT character on television.

Could Vicious, then, despite its disappointing original title of Vicious Old Queens, open the eyes of TV commissioners and start a new trend where characters happen to be gay, rather than are typecast as “gay characters”? It is something that LGBT charity Stonewall hopes to see.

“Many gay people, especially those who’ve been in relationships for many years, will be pleased to see their lives reflected in the characters in Vicious,” a spokesmen for Stonewall said.

“Sadly there are still too many examples of programmes poking fun at gay people, or suggesting that we’re almost entirely defined by our sexual orientation. Gay people come from many different walks of life, and it’s important that this reality is made more visible on TV."