A judge has ruled Britney Spears out of testifying in her former manager Sam Lutfi's ongoing defamation battle with the superstar's family.

Lutfi, who was fired as Spears' manager in 2008 as the pop star's life unravelled following a breakdown, requested that the Toxic hitmaker undergo evaluation to prove she was mentally capable of testifying in court in his legal wrangle with her mother Lynne Spears.

He filed suit against Lynne over allegations she made in her memoir Through the Storm: A Real Story of Fame and Family in a Tabloid World. In the tome, she claimed Lutfi secretly drugged her daughter and refused to let her freely speak to family and friends.

But a judge has now ruled that the pop star, who is still under a conservatorship order following her much-publicized meltdown, will not have to testify when the trial starts next week (begs01Oct12).

E! News reports Los Angeles Superior Court Judge Reva Goetz, who has been overseeing Spears' conservatorship case, ruled that putting the star on the stand would cause her "irreparable harm and immediate danger".

Spears' fiance Jason Trawick is on the witness list and the singer's mother and father, Jamie, are also expected to take to the stand. Trawick and Jamie Spears are co-conservators of the singer.