Britney Spears sat in the blacked-out SUV in the hospital car park. For two hours she waited patiently as her boyfriend, Kevin Federline, witnessed the birth of his second child Kaleb.

When the baby arrived, Spears and Federline returned to the superstar’s LA home – while Federline’s then-fiancee Shar Jackson was left alone in her hospital room holding her newborn son.

In one of the biggest bombshells in her new memoir, The Woman in Me, released this week, Spears insists she was ‘clueless’ about the fact Federline had a pregnant fiancee and was already a father to a two-year-old daughter when they met in 2004.

She writes: ‘I was living in a bubble and I didn’t have a lot of good, close friends to confide in and get advice from. I had no idea until after we’d been together for a while and someone told me, “You know he has a new baby, right?”

But today, in an exclusive interview, Federline’s ex-fiancee Shar Jackson blasts Spears’s claim, alleging that not only did the Toxic singer take Federline from her, she was fully aware of the pregnancy and even waited outside the hospital in Orange County, California when Kaleb was born.

Speaking for the first time about the heartbreak she went through, former child actress Jackson, 46, said: ‘I felt betrayed and crushed. I thought, ‘How could you do something like that?’

The, star of a 1990s US sitcom Moesha, Jackson had been with back-up dancer Federline for two years when he met Spears at Los Angeles nightclub Joseph’s in April 2004. Jackson said: ‘When Kevin met Britney we had three kids [Federline was stepfather to her two eldest children and they had a daughter, Kori, together] and one on the way and we were living in a six-bedroom house in Orange County, California. One day, when I had bad morning sickness and was about six months pregnant, Kevin had an audition for a coke commercial or something in LA… then I didn’t hear from him for three days.

‘I later found out they had met at a club on the first night, when he went out with his friends. I felt angry and betrayed and I went numb. I went onto autopilot.’

In her book Spears, 41, insists Federline, 45, ‘did a number’ on her by concealing his secret family.When a friend eventually told her Jackson was pregnant, Spears says she confronted him, writing in her memoir that she yelled, ‘You have kids? You have children? Not only one child but two children.’

Jackson had first crossed paths with Spears when the singer was dating Justin Timberlake. In another bombshell from the book Spears writes she felt ‘forced’ to abort Timberlake’s baby and that he later broke up with her via text.

Jackson said: ‘I met Britney way before Kevin got with her. We ran in the same circles and I was good friends with the guys from NSYNC [Timberlake’s boy band]. I thought she was cute and adorable. I wouldn’t say Britney and I ever became close but we were always respectful and cordial to each other.’ Jackson was madly in love with Federline after meeting him at a dance studio. ‘We moved in together after about a year and got engaged right after our daughter Kori was born,’ she said. ‘I was very much in love. Not only was he gorgeous but he took my two older kids under his wing. I was like, ‘Oh my God, he’s perfect.’

Even after she discovered Federline was cheating on her, Jackson says she fought to save their relationship. ‘We didn’t break up, we agreed to figure it out. Then he told me he had booked a commercial and needed to go and shoot it overseas.’ In fact, Federline met Spears – and pictures of them embracing on a beach appeared in the newspapers. Jackson said: ‘For about a day-and-a-half I was in a state of disbelief. Then I called him and said: “I need to know if you are going to be in the kids” lives or not, because it’s not a part-time gig. If you don’t want that responsibility, tell me now so we can cut ties completely.’ I didn’t care about Britney at that point. It wasn’t about her. I felt betrayed and crushed. I thought: “How could you do something like that?” ‘

Federline never returned to the family home. He and Spears became engaged in July 2004, just days after Jackson gave birth to Kaleb. Federline and Spears married that September and had two sons – Sean, now 18, and Jayden, now 17 – before separating in 2006.

In her book Spears writes: ‘When I married Kevin, I meant it with all my heart.’ But Federline became ‘enthralled by the fame and power.’ ‘Again and again in my life I’ve seen fame and money ruin people,’ Spears writes. ‘And I saw it happen with Kevin in slow motion.’

Despite everything, Jackson remains close to Federline, who now lives in Hawaii with Victoria, his wife of ten years, their two children and Sean and Jayden. And Jackson holds no ill-will towards Spears. ‘I think her relationship with Kevin was a fantasy for both of them. To me, anything built on a rocky foundation isn’t going to last and they started rocky because he was in a relationship. I never hated Kevin because if you really love someone you can’t hate them.’

Representatives for Spears and Federline were approached for comment.