Los Angeles: She may be a chart-topping artiste with millions in the bank but as far as Britney Spears' personal life is concerned, it refuses to spring back to normalcy. Her rivals have already created hue and cry about her mental state and now her parents have added to the former’s credibility by stating that their daughter is "mentally incapable" of testifying in a court case against her mother.

The popstar's mother Lynne is being sued for defamation by Spears' ex-manager Sam Lufti over claims made about him in her tell-all memoir, 'Through The Storm'.

She alleged that he regularly drugged Spears and harassed her following her well-publicised breakdown three years ago.

When Lufti requested that Spears be called as a witness to testify in the case, Lynne claimed that her 29-year-old daughter is incapable of doing so, according to Hollywood Reporter.

The singer remains under the conservatorship of her father, Jamie Spears, and her lawyer, Andrew Wallet.

Both are unwilling to let Lufti have contact with Spears, who they say is in a "fragile mental state".

But Lufti has rejected their claims, arguing that Spears is now back in control of her mental health.

According to the magazine, Lufti has had an expert in psychiatry analyse a recent video interview with the singer in support of his argument.

"In the Ryan Seacrest interview, which was recorded approximately two months ago, Britney Spears is interviewed at length, and she responds logically and coherently to questions, evidencing logical thinking and mental competency," Lufti told the court.