The troubled 'Toxic' singer mysteriously pulled out of recording the track, which was due to be produced by Timbaland, with her former boyfriend last month.A source told the New York Post newspaper: "Timbaland set aside a week out of his crazy schedule to do this - and then, just before she was supposed to fly out, Britney abruptly cancelled the session and refused to do the song.
"It's crazy. She's looking for a comeback, and this would have not only been a huge hit, but something she could have opened the MTV Video Awards with it and really blown everyone away."
The duet was set to be recorded at Timbaland's Virginia Beach studio and Justin reportedly wrote the track especially for Britney.
Britney's record label Jive is said to be worried about the star's erratic behaviour and had hoped the duet would resurrect her faltering pop career.
Another sourced added: "Everyone is worried. In her mind, her album is done and she's done enough work. She's an easy target right now, because she's sick. She needs help. It's sad because what she's got - and we've heard it's like bipolar disorder - can easily be treated with medication, but she won't do it.
"People at Jive have been with Britney since album one, and have an investment in her. They have faith that once the material is there, she will become focused and step it up. But at the end of the day no-one can do it for her."
Representatives for Jive and Justin declined to comment.
Earlier this year, Timbaland approached Justin to help propel Britney back to the top of the charts.
He said: "I feel her pain, it really bothers me. I asked Justin, 'How would you feel about me working with Britney? Would you do it with me?' He said he would."
Britney and Justin dated for four years until 2002.