When I meet the novelist Sarah Waters, I wonder how quickly we can get on to discussing the l-word. Because the point about Waters is that she’s not just a stunning historical novelist who animates her period settings with gripping Gothic stories and properly conceived characters, but she’s a stunning historical novelist with a lesbian perspective.

Her novels Tipping the Velvet, Fingersmith and Affinity (the so-called “Victorian” trilogy) all involve quite a bit of Sapphic love. Indeed, BBC America felt it had to censor the television adaptation of Tipping (the title is a Victorian synonym for girl-on-girl action) though the BBC here seemed to think it was perfectly all right and left it intact. Waters’s follow-up novel – The Night Watch, set during the London blitz – was runner-up for all sorts of literary gongs, but since there were racy female scenes in that book, too, the sex rather hogged the limelight.

Fortunately, Waters doesn’t seem to mind the lesbian label too much. A slender blonde of 42, she has a merry face and an easy smile. Within five minutes of meeting her, I feel I can ask her almost anything.

“Writers like Jeanette Winterson have resisted the lesbian label, but it’s never felt like a problem to me,” she says. “I’m very lucky. I have a lesbian audience but a mainstream one as well.”

In a marketplace where literary writers are stamping over one another to distinguish themselves, Waters has carved out a wonderfully clear position for herself. In her own words: “I use a period landscape we all know well, and put lesbians in it.” What a fantastic notion! Waters nods happily. “I think it makes for quite an interesting experience – for straight and lesbian readers alike – to go back into the past, and think, ‘Oh yeah, it’s not actually all heterosexual’.”

As she points out, gay male literature reached this point some years before. “Gays have such an obvious historical record – think about Oscar Wilde,” says Waters. “I think this is because male homosexuality was illegal; men were arrested for it, executed for it. Whereas women were under the radar a little. In a way, it was always easier for the mainstream to ignore them.”

Which makes her position all the more unassailable, since there is no tiresome lesbian literary hinterland to keep bumping into.

As a former PhD student, researching gay and lesbian literature, Waters knows exactly what there is to study from that period and it’s not much. “In a funny sort of way, that was quite liberating because it gave you room to make things up,” she says.

The lesbian content of her novels has an understandable appeal to a gay audience, but how does she explain the fascination of her considerable fan base of heterosexual read-ers? “Well, I know many women who are very, very interested in gay male sex, and will read male gay porn. Only it’s not very acknowledged.” Right. Um. Back to the books, though.

“I’m sure there’s a sort of prurient appeal, definitely. Lesbians have always had that role in porn, rather as if they are served up as a hors d’oeuvre before the main action. At the same time, the books resist that sort of lure because the man doesn’t step in after the ‘fore-play’ of lesbian sex. But the novels are also about other things: betrayal, loss and so on – things everyone experiences.”

Indeed, Waters is now considered so mainstream that Affinity, her Gothic ghost story set against the backdrop of the Victorian women’s prison at Millbank in London, has just been filmed for ITV1. As with Tipping the Velvet, the adaptation has been handled by Andrew Davies, the screenwriter who famously put Colin Firth on the Alist by virtue of a wet shirt.

“He feels the great 19th century writers would have written about gay sex, if the mores of the day had allowed it,” says Waters. “So in a sense he is just doing what they would have done.”

Davies clearly loves a good bodice-ripper – but Affinity is actually nothing of the kind. A literary tour de force, its narrative animates the thoughts of Margaret Prior, a young spinster obsessed with a spiritual medium who is locked in prison for most of the book. Prior’s passion for the bewitching medium – which is real though unlabelled – exists chiefly in her mind.

Undeterred by the lack of wild sex scenes, Davies was so keen on Affinity, which he considers Waters’s best book, that he tried to raise interest in Hollywood to have it made into a film. As it turns out, the novel’s intimate and claustrophobic atmosphere is perfectly suited to television, even if its suggested lesbianism has – of course – been made more explicit in the transfer to the small screen.

Does Waters enjoy writing about sex? “I do. It’s so easy to sound corny, so it’s interesting to think how to do it in a way that sounds fresh and authentic.” Does she use personal experience to help her? “I don’t know. Most of my sex scenes have a weird, extra charge because they are Victorian. That’s part of the project, in a way: to set the stories at a period when lesbian desire, or all sorts of desire, is pretty subterranean, so . . . no, that’s not my experience.”

Waters, who now lives with her lover Julie in southeast London, admits she took years to “come out” to her parents – “but parents are a special case”. Growing up in Pem-brokeshire during the 1970s, she was a self-confessed “horrible swot” who was “completely into boys”, yet had a lingering sense that she might be bisexual. It was only when she went to university that she fell in love with another woman – at the time probably more of a statement than it would be now.

“The lesbian aesthetic has changed an awful lot from that which I grew up with in the Eighties and Nineties,” says Waters. “When I was coming out, being a lesbian was political; it was about feminism. It’s more detached from that now; it’s more about a lifestyle thing, fash-ion-based and looks-based.”

Shocking rumours are circulating that her next novel, due to be published next summer, has no lesbians in it at all. Can this be true?

“It’s just the way the story turned out,” she says. “There was an angry letter to the UK lesbian magazine Diva from a reader who thought I’d betrayed lesbians. She said she wasn’t going to buy my books, and neither were her friends. Which was a bit depressing and made me feel a bit crap.”

But not for long. Waters is sufficiently well established to venture away from the theme that made her name, for one book at least. But she’s not going to abandon her main theme for someone else to harvest. “The book after the next one, I’ll come back to a lesbian story.” Phew.

Affinity will be shown on ITV1 at 9pm tonight