The 34-year-old became the youngest singer to ever debut in the U.K.'s number one chart position with her classic smash hit Because We Want To. But the mother-of-two admits she remembers little of those days in her life.

"What's even more alarming is that I can't even remember (those years)," Billie explained to the Evening Standard's ES Magazine. "It wasn't like I was out, getting wrecked.

"It was because I must have just shut it out. Which is probably because it was on some level deeply traumatic. Not because I was some horrible child slave, I just worked so hard. I have never worked as hard as I did in those years."

As well as her success in the music industry, Billie has also found fame on the small screen, with one of her most famous roles that of Rose Tyler in Doctor Who between 2005 and 2013, opposite David Tennant as the Doctor.

While the programme led to a huge revival in Billie's career, it also had its downsides - namely that she couldn't go out without being recognised again.

"It makes you so famous, a show like that," she added. "It gave me little memories of being a popstar - you can't go anywhere without it being a thing, and I found that quite hard again. But it changed everything for me. And I have never worked with a nicer bunch of people in my life since."

As well as Doctor Who, Billie has appeared in television shows such as Penny Dreadful and Secret Diary of a Call Girl. In the latter, she played an escort called Belle, and the programme led to her being inundated with work offers which involved her stripping off.

"What's annoying is that they are fun roles, if you can remove the sex," she explained. "It's the sex that makes it annoying. Otherwise they are interesting stories, interesting women with chequered pasts."