From riches to rags: Katherine Jenkins' 60-minute makeunder

This busking lark is pretty raw stuff for Katherine Jenkins. Generally she embraces a Miss World breed of glamour that speaks the international language of fame. Today, though, as she descends the escalator into the bowels of Leicester Square Tube station, she is in a raggedy red jumper, black trousers tucked into unexpectedly cool boots, and a brown wig. This is not her mufti, you understand, it is a styled civilian get-up, and the argument for the wig is that there should be an element of disguise.

Always a pretty girl, the 31-year-old looks even better as a brunette. As a blonde she is rather too painted-doll: peaches and cream, but brittle. Even though the wig is clearly plastic, the wearer looks somehow less plastic. But she is in full make-up, with her people hovering for touch-ups, which I would imagine is unusual for buskers.

But Katherine Jenkins is a brand. And for a woman who has sold seven million albums, who tours the world bringing tears to people's eyes with her renditions of 'I Dreamed a Dream' (in French) or 'I Will Always Love You' (in Italian), who went straight from the Royal Academy of Music undergraduate course to an unheard-of six-album deal and who published her autobiography at 27 (which, compared to Justin Bieber or Wayne Rooney, is positively prehistoric), busking at the neck of the Northern Line is certainly pared down and perhaps rather intimidating. She made £16 in 40 minutes, and of the commuters who stopped and listened at least three, who from the evangelical light in their eyes looked like die-hard fans, had tears in their eyes. She does, it must be said, make a very nice, safe, pretty noise.

'I'm never nervous when I sing,' she says. 'Ever. But this was strange because I thought, "How are people going to react?" It was the feeling of being completely alone. Normally if I'm on stage then I'm working with my conductor or I've got a sound man' (there is a sound man hovering in a corner, twiddling his knobs) 'but this was just me turning up on my own and singing. It was really back to basics, but I loved it. When I go to Iraq or Afghanistan, it's just with a backing track and you have no idea what the acoustics are going to be like because it's a makeshift space in an aircraft hangar or something. So it's a rough-and-ready thing and this was a similar sort of attitude. I had no idea what it [the busking] was going to be like but I tried my best.'

I'm sure the troops have pictures of ladies far less genteel than La Jenkins pinned to their walls, but her morale-boosting trips have been a great success. She has been called a latter-day Vera Lynn. Perhaps it is that intensely ladylike quality that reminds so many people of what now seem like better times. And just as the light quality of her operatic voice compels the cross-generational hordes who are otherwise indifferent to much music, so it offends classical purists (David Mellor called her 'vomitacious'). There's also been a cyber-bullying incident recently, more of which later. Anyway, despite all the accusations that she is not a proper singer, that she is not fully trained, that she is - in the eyes of the hardcore classicists - a bit of a disgrace, she is a hard worker and a roaring, runaway crossover success story.

It is a story that began in the Welsh town of Neath, near Swansea. 'I've read before that it was a tough council estate and that irritates my mum because it's really not,' she says. 'It was a nice place to live and I wasn't doing any of that hanging out on the street, but even if I had, that would have been quite innocent in itself.' Katherine's father Selwyn was a factory worker who was in his mid-fifties when she was born and he took early retirement when she was six and her sister Laura was four. Her mother Susan returned to work as a radiographer and so it was Selwyn who rushed around after the girls. 'He made our tea and did our lunchboxes and drove us to and from school. I think it was quite modern of him,' says Katherine. 'He was an older dad, laid-back and worldly, and I think he passed some of that easy-going stuff on to me.'

It turns out that Selwyn had twin daughters, Pauline and Angela, from a previous marriage. They will be 60 next year. 'I didn't really know about them until it was in the Daily Mail the other day,' she says. 'I don't know them and I don't really like talking about them because it brings them into a world they didn't ask to be part of. They are apparently very nice people. I genuinely don't know much about it because my dad is not alive to ask.'

Selwyn died of lung cancer when Katherine was 15. He was ill for only two months and Katherine, who had been singing two services a week in the local church since she was eight, struggled with her faith. 'I was really angry with God but I wanted to do something with my life for my dad. Losing somebody is a reality check and my attitude to life is to take every opportunity.' Which she did. Excellent GCSE results, four A levels and a scholarship to the Royal Academy of Music.

It was during this time - between being Welsh Choirgirl of the Year and becoming the fastest-selling mezzo-soprano in history - that Katherine seems to have had some non-genteel fun. Someone from her student days apparently has a video of her glassily telling the camera, 'I am f***ed.' Before that was leaked, she gave an interview to Piers Morgan listing the drugs she had taken at college: cocaine, ecstasy and cannabis. 'It's a hard one,' she says. 'When you are a classical artist, people assume you are saintly and that you haven't lived.' An insider confirms that although classical musicians are more punctual than rock stars, they are just as dissolute, but Katherine is quite correct, that is not the general perception. In fact the headline was: 'I took drugs and I'm ashamed, reveals heavenly soprano Katherine Jenkins.'

So, whether or not she knew about the compromising video, she chose not to completely whitewash the drugs story. But she does seem to have whitewashed herself. She is amazingly hard to reach. It is frustrating because she's terribly nice, but a bit blank. Is she a simple Welsh girl just singing her way through life, or a determined machine ploughing onwards with a huge, bleached grin? Whichever it is, I'd rather hear her sing than talk.