Sherine Ebadi claims that Jamie Spears engaged in "unconscionable violations" of Britney's privacy in her bedroom, and had asked others to do the same.

A listening device was said to have been concealed behind furniture in the singer's bedroom, and fitted with battery packs to allow it to record for longer.

Testimony for Ms Ebadi in court documents seen by the Press Association, suggests that Mr Spears had asked asked company Black Box Security, the firm in charge of protecting Britney, to spy on her.

A former employee of the company, Alex Vlasov, backs up the allegations, Ms Ebadi claims, saying: "Based on my debriefing of Mr Vlasov, I have also corroborated that Mr Spears instructed Black Box to place a secret recording device in Ms Spears's bedroom.

"At the time, Mr Vlasov observed that the digital recording device had an SD card, a battery pack attached to it, and was covered in duct tape.

"According to Mr Vlasov, Black Box was initially responsible for suggesting that a secret listening device be planted in Ms Spears's bedroom, but Mr Spears 'loved' the idea and approved and instructed that the installation move forward.

"The Black Box employee who placed the secret device in Ms Spears's bedroom explained to Mr Vlasov that he did so by duct-taping it behind furniture so it could not be seen, and that he added a separate battery pack to the recording device to permit continuous recording for a longer period of time".

Ms Ebadi added that California is a "two-party consent" state, meaning that everyone involved has to agree to confidential conversations being recorded, and that Mr Spears had instructed the company to listen to others, using methods that are "typically only available to law enforcement".

"Mr Spears... engaged in and directed others to engage in unconscionable violations of Ms Spears's privacy and civil liberties, which also implicates federal or state criminal violations of the law," the former agent said.

Jamie Spears has called the allegations "all nonsense".

The controversial conservatorship that saw Spears' father control the star's personal and financial affairs for 13 years came to an end in November following a court ruling.