Los Angeles, California - A former FBI agent has concluded that Britney Spears' dad used a secret listening device hidden in her bedroom to spy on his daughter during the years of her conservatorship.

Sherine Ebadi said Jamie Spears had engaged in "unconscionable violations" of the pop star's privacy and civil liberties, and had encouraged others to do the same.

A device was allegedly duct-taped behind furniture in Britney's room so it would not be seen and was fitted with an extra battery pack to allow recording for longer periods.

Legal documents obtained by the PA news agency show testimony from Ebadi in which she says Spears had directed Black Box Security, the company in charge of protecting Britney, to spy on her.

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Ebadi said she had corroborated the information after having spoken with whistle-blower Alex Vlasov, a former Black Box Security employee.

"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," she said.

"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."

California is a "two-party consent" state, which requires that all parties consent to any recording of confidential conversations, Ms Ebadi said.

She said that Spears had also instructed the company to surveil "individuals of interest" to him using methods "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 federal agent said.

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The Toxic singer was freed from the controversial conservatorship arrangement, which allowed her father to control her life and finances for nearly 14 years, in November.

Lawyers for Spears have called for his daughter's estate to continue paying his legal fees, saying he had "stepped up" and kept "opportunistic and wholly self-interested parties at bay".