The 'Toxic' singer - who had months earlier entered rehab after shaving her head - had been planning her routine at the prestigious event for months to prove to the world she had fully recovered from her personal problems.
However, insiders have now revealed she wasn't mentally ready to perform and was drunk when she went out on stage.
In his new book 'Britney: Inside the Dream', writer Steve Dennis claims: "Britney herself managed to ruin the most important performance of her life. Hours before the show, her anxieties raged. She decided it was a good idea to start drinking tequila shots to calm her nerves - one after the other."
Britney's fragile mental state was also reportedly tested when her ex-boyfriend Justin Timberlake came into her dressing room to wish her luck.
Dennis revealed in the book: "Justin was cool and couldn't have been friendlier, but that wigged her out more."
Her preparations descended into chaos when Britney allegedly refused to let celebrity hairstylist Ken Paves attach her hair extensions.
When Britney told Ken to leave, her staff began desperately searching for other hair extensions - which were needed because her natural locks were still very short - and they had to quickly find some from her hotel's salon.
When the mother-of-two did appear on stage - dressed in an unflattering black underwear set and fishnet tights, after reportedly refusing to wear the agreed more flattering one-piece - she looked dazed, forgot some of the lyrics to her song 'Gimme More' and barely joined in with the dance routines.
The performance was ridiculed by many critics and the book claims Britney - who was going through a messy divorce and custody battle with ex-husband Kevin Federline - was in tears for 45 minutes after leaving the stage.
The months after her chaotic comeback saw her lose custody of her two young sons, Sean Preston and Jayden James.
In January 2008, was taken to a psychiatric ward after holding Jayden hostage in her home.
She has since turned her life around and at the 2008 MTV Video Music Awards she looked a picture of health and collected two prizes.
She is currently performing in 'The Circus Starring Britney Spears', her first concert tour in five years.
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