In her first interview since the break-up, the teenage chart-topper confesses: "I wasn't aware that a split was around the corner. We'd been going out for about a year and a half. We didn't see a lot of each other, though, so that took its toll and it just fizzled out. But I loved him and thought our love could conquer that. He rang up last week and said he didn't think it was working and I suppose I agreed. We decided to end it and there were a few tears. I still feel empty inside but you never know, we might get back together in a few years when we have more time. I feel numb and heartbroken but luckily I haven't got time to sit down and dwell on it. I am gutted, though, and it'll be really awkward the next time I see him because I won't know what to say. I am missing him. It's like he's still away and I'm just waiting for him to come home. I used to speak to him at night before I went to bed and that won't happen any more - that's what's difficult. I'm not on the lookout for anyone else and just want to recover for a while. We had so much planned, like a joint birthday party together in September, but I'll have to have it on my own now. It's very sad. There's certainly nobody else as far as I'm concerned and I hope that's the same for him."

Ritchie may well come to regret his decision. Billie, who will be 18 in September, has transformed herself from little girl to beautiful young woman. The Sun first revealed the two pin-ups were dating in March last year. Only days before the split, Billie was telling me how she loved the idea of marriage. She did not stop talking about Ritchie. But she did admit their love life was under threat from their hectic schedules. When Billie topped the charts with Because We Want To in July 1998, she was the youngest female to have a UK No.1 for 38 years. Ritchie helped her cope with the intense pressures that engulfed her. For Billie quickly learned that success has its downside. Her growing wealth and familiar face was attracting hatred and jealousy. Going out shopping in her home town of Swindon suddenly became a nightmare. Abuse was screamed at her in the street and she has been barged by girls who envy her success as the British Britney Spears. And she will never forget the moment of horror when she received a letter containing the threat: "You are going to die."

She tells me: "There were a series of really horrible letters saying they wanted to kill me. Some of them were addressed to my mum and dad, so it was really distressing for them too. It's a chilling feeling when you open an envelope and you read something like that. It scared me a lot and sent shivers down my spine. That's why I am always very careful and look over my shoulder. I keep my head down, particularly when I'm at home. I get quite a lot of mouthing off and it is hard for me to go out. It's mostly women and can get very bitchy. There are a lot of girls who elbow me when I walk past them. Most of my problems are with women because I think they were very jealous of my relationship with Ritchie. They think that I stole their heart-throb away from them. I know how much they love him. It's weird being in the public eye as a teenager. You do make mistakes but I haven't turned my back on my roots. My parents have become my best friends but I live alone and do my own cooking, washing and cleaning. I'm quite responsible and can't ever imagine living at home again. I like to escape and be on my own. I'm just going to try to enjoy being single for a while. It seems ages ago that I was 16. So much has happened. I've changed a lot, grown up a bit, experienced a lot. I was straight out of a small town, from youth clubs and dance classes. I'm not that different inside. I've had a taste of reality."

Billie's mum Mandy, 40, and builder dad Paul, 42, look after her finances and give her an allowance. She does not drive yet and her biggest extravagance is a £200,000 flat in West London. Her new single Night And Day looks like repeating the success of Girlfriend and Because We Want To and is very much in the Britney Spears mould. Now Billie has tasted success she doesn't want to lose it. Relaxing in her record company HQ in West London, she takes a sip of Diet Coke and says: "I'd be gutted if I lost all this and had to go back to normal life, though. If my comeback record fails I'll be devastated - but I think I will be around for a long time to come. I admire Madonna and she is probably my role model. What people have to remember is that I am not a little girl jumping around in combat trousers any more. But I'm not ashamed of what I was like."

With that, she gets up up to go and guess what she's wearing... combat trousers.