She awoke in her tent, alone, with no memories of what had happened as Johnston "talked with his friends on the other side of the canvas". She had vowed to wait until marriage. And she had lied to her parents about where she was going.

Palin, a 20-year-old single mother and the daughter of former Alaska Republican Governor Sarah Palin, tells a story of "deception and disappointment" in the book, Not Afraid of Life: My Journey So Far.

Bristol Palin and Levi Johnston appeared on the cover of Us Weekly.

The memoir, co-written with Nancy French, is scheduled for publication by William Morrow next week.

Palin's book covers growing up with her family, which she portrays affectionately, and the excitement of her mother's political life as governor and then, in 2008, as the GOP vice-presidential candidate. But the main theme is her on-and-off-again relationship with Johnston, with whom she had a child and was briefly engaged while caught in a media spotlight.

She blasts him as "the gnat named Levi Johnston constantly spreading false accusations against our family" and calls him a self-involved slacker "who cheated on me about as frequently as he sharpened his hockey skates".

But Palin, self-described as a good girl and straight-A student, had been drawn to him and his bad-boy manner from the time they met in seventh grade.

When she confronted him about their sexual encounter, he said what she wanted to hear: They wouldn't do it again until they were married.

It didn't work out that way, though Palin notes that when she got pregnant she was on birth control pills prescribed to treat her cramps. Only by the eighth home pregnancy test, she writes, was she convinced of the positive results.

When she told her parents, they were accepting, not condemning, she writes, and focused on the future, particularly her continuing education.

Johnston's reaction wasn't so reassuring: "Better be a [bleeping] boy," he declared.

Palin writes of being awakened early one morning in August 2008 by her father, Todd Palin. He gathered the kids, collected their mobile phones and announced they were going on a surprise trip.

The destination was Dayton, Ohio, where, the next day, Arizona's US Senator John McCain, the GOP presidential nominee, introduced his vice-presidential pick and her family to the world.

By then, Sarah and Todd Palin knew their daughter was expecting. The public found out during the GOP convention, when Sarah Palin was shocked to see a TV network carrying an unapproved message from her commenting on it.

The entire family, along with Johnston, appeared at the convention.

Bristol Palin writes that, in her mind, being there on display with Johnston "somehow legitimised us as a couple."

But soon after Tripp's birth in December 2008, Palin writes that she found a text on Johnston's mobile phone that revealed he had been cheating on her again. He confessed and left the family's house.

They reconciled one more time 18 months later. Johnston presented her with a ring, and they decided to sell the story of their engagement to Us Weekly magazine. But by the time the story appeared, they had split for good. Johnston had informed her he had gotten another woman pregnant.

"I'd just made a complete fool of myself and given my family the middle finger," Palin writes of hearing the news.

Johnston cashed in on his newfound fame in ways Palin found appalling. Her reaction to learning that her son's father had posed nude for Playgirl: "Puke!"

But Palin's unsought celebrity brought her opportunities, too. Last autumn she competed on ABC's Dancing With the Stars, where, she writes, as each week passed and she survived her rivals "got a little colder".

In her book, Palin describes herself as "just a normal girl who couldn't hide her problems and learned a few lessons along the way".

She has since emerged as a spokeswoman working to prevent teen pregnancy, earning thousands of dollars, which she doesn't discuss in the book. In addition, the Bio Channel has announced a reality series in which she will star.

Meanwhile, Johnston, who has feuded often with the Palins, has promised to set the record straight in his own book, Deer in the Headlights: My Life in Sarah Palin's Crosshairs. It's scheduled for publication in the northern hemisphere autumn.

AP