The early influences on Katherine Jenkins explain almost everything. There was Maria Callas, obviously, but there was also, right at the other end of the range, Marilyn Monroe. To be charitable to her, she was a film star who, as Noël Coward said of himself, couldn’t really sing but knew how to. The lack of virtuosity hardly mattered. Glamour was what she brought to the party and this she did in truckloads. Down in the old South Wales steel town of Neath, the young woman who is now its most famous daughter saw her on TV singing Diamonds Are a Girl’s Best Friend, and was hooked.

But on what exactly? “Well, it was not so much me thinking, wow, what a talent,” Jenkins explains. “It was the sheer style and femininity, those were the things that captured me. I adore the fashion of that time, and to me she was, and still is, the very image of Hollywood stardom.” Now consider that Jenkins took the rigorous classical route, joining the Royal School of Church Music Cathedral Singers, twice winning the BBC Radio 2 Welsh Choirgirl of the Year contest, and getting a scholarship to the Royal Academy of Music in London. Once you know that she has been running these parallel romances with the classical and the populist from an early age, it makes sense that she should have emerged as the queen of crossover. Even as a student she was confounding the stereotype of the diva by working as a model to help with the rent. At 27, she remains trimly photogenic, and as popular a choice of mascot as the Welsh rugby team could have made.

The statistics are overwhelming proof of her status. All her studio albums – four in as many years – went to the top of the classical charts. In the course of this, she became the only artist to hold the No 1, 2 and 3 positions at the same time. Her first one, Premiere, was the fastest-selling by a mezzo soprano. Her fourth, Living the Dream, stayed at No 1 for almost a year after its release in 2005.

But even more remarkable than this was its rise to No 4 in the pop album charts. Here was something other than the next Girl With A Great Voice; she was a creature from the other side of the tracks, a proper classical mezzo with a power that had once shattered part of a chandelier. Here too was a record with Handel, Bach, Satie, unpronounceable Welsh ( Cymru fach) and Italian ( Lascia ch’io pianga) airs, going toe-to-toe with Girls Aloud and Coldplay.

Crossover, the realm she reigns over, can be seen in different ways. There is the phenomenon of Pavarotti’s Nessun Dorma reaching No 2 in the UK singles chart (highest classical placing) through exposure as the theme tune of the 1990 football World Cup; or Mozart’s Piano Concerto No 21 selling well on the back of the film Elvira Madigan in 1967. These are the pieces that deservedly get lucky rather than set out to compromise themselves for commercial gain, and Jenkins’s success belongs in this first category.

Then there is the phenomenon of classical musicians choosing to perform popular numbers, for example, the Three Tenors doing My Way. Jenkins belongs in this category, too, and never more so than on her new album Rejoice, which has songs written and produced for her by established commercial operators. These include Steve Mac, who has written for Westlife and Toni Braxton; the Take That singer Gary Barlow; and Simon Franglen, whose songs have been recorded by Céline Dion and the Bee Gees.

There are songs about love and losing, which she accepts have a poignancy for her because of her split last year from her longstanding boyfriend Steve Hart, a songwriter and former member of the Worlds Apart boyband. Their careers were getting in the way, she says. “It was hard. We were both so busy. I’ve said to myself that I’m purposely not going to meet anyone at the moment.” Has she been as successful in this as in the rest of her life? “Well, I’m single, so yes.” For present purposes she is part of a very singular double act with the great Darcey Bussell, whose retirement is proving operatic. Five months after her emotional farewell performance for the Royal Ballet at Covent Garden, she and Jenkins are co-starring in a stage show, Viva la Diva, a highly eclectic homage to their own heroes and heroines.

“Darcey and I have been friends for about two years,” the singer explains. “We were always being asked to the same events, and so we kept on meeting. She knew she was going to leave the Royal Ballet, and my manager had the idea of us doing something together. Like me, her influences have been very broad, so that while mine go from Callas to Doris Day, hers go from Fonteyn to Fred and Ginger. We’re not attempting to do impersonations of them, just to pay tribute to them and their achievements.”

Not everyone approves of Jenkins and what she is up to. In a Britain allegedly tearing down old cultural barriers between High and Low, there are reliable pockets of snobbery in both directions. One of the criticisms of her is that she has never sung opera, and would be exposed as underpowered if she were to try. There is a semantic nicety here, for while she has sung countless arias, she has never performed in a full-length stage opera. She leaps with surprising alacrity to the side of her critics.

“I agree, I agree. I’ve never actually called myself an opera singer. Classical, yes. But I’m too young to be doing full opera. I’m still training with my teacher from the Royal Academy, and we are getting there. Give it three years. And then, I’m sure it will be, what can I say . . . up for discussion by the critics.”

Ask her whom she admires these days and there’s no stopping her. None comes higher than Plácido Domingo, who has become “a great friend and inspiration”. Pavarotti too, with whom she was due to sing. But then there’s Whitney Houston, particularly the early recordings, “so warm and beautiful”. And Dame Vera Lynn, whose successor she has become as Forces Sweetheart through her enthusiasm for entertaining British troops abroad. And Charlotte Church, who has single-mindedly taken the raunchy route. “I’ve warmed up with her back-stage, and it’s a tremendous voice.”

Then there’s Prince, who so bowled her over that she went twice to his show at the O Stadium. Also her compatriot Shirley Bassey, that most operatic of nonopera singers, whose hit I Who Have Nothing Jenkins sings on Rejoice. It’s a bold move, putting a decorum into a number that Bassey’s magnificent melodrama had turned into a sort of hysteric’s anthem. Such are the surprises in Jenkins’s style.

Whatever happens to the opera career, she isn’t about to stop making her own choices and watching unrepentantly as great and classic songs encroach further into the popular consciousness. It’s hardly something we should be getting, well, cross over.

Rejoice is released by UCJ. Viva la Diva tours from Sunday until December 21 2007 (www.katherinejenkins.co.uk)

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