Britney Spears is terrified of her father Jamie after he allegedly threatened to stop her seeing her sons if she disobeyed him, a California court has heard.

The singer was so scared of her dad that she requested a secret phone to call her former 'manager' Sam Lutfi, his sister Christina told Los Angeles Superior Court on Wednesday.

She was speaking in defence of Lutfi, as a court determines whether to extend a restraining order against him.

Jamie Spears, who has court-ordered control over her estate which Sam's lawyer referred to as a "prison", sought protective orders on her behalf against Sam, as well as her ex-boyfriend Adnan Ghalib, in January.

Court documents suggest the men attempted to undermine Jamie's conservatorship, established after the singer suffered a series of public meltdowns at the beginning of last year.

But Christina Lutfi is adamant her brother was only trying to help the star.

She alleges Britney secretly enlisted her help as she was worried she would lose access to her two young boys by ex-husband Kevin Federline, three year old Sean and Jayden, two.

The claim comes just days after a voicemail the singer allegedly left for her legal team was leaked online, in which she states: "...My father has threatened me several times, you know, he'll take my children away."

Christina told the court, "(Britney) told me she wanted to get a hold of Sam. She wanted him to help her find a lawyer, and wanted someone to get a prepaid cell phone to her. She was scared because her father was blackmailing her with visitation rights over her kids."

She arranged to meet the star in a hotel sauna and gave her a secret mobile phone, when Britney allegedly asked her, "Will Sam be able to help me?"

The cell phone was later discovered by the pop star's security team and taken from her.

The court had earlier been told by Blair Berk, a lawyer for Jamie, that the singer "was in such fear (of Sam), she asked if I could get him arrested, to keep him away from her."

But Sam's lawyer Bryan Freedman tells People.com that his client's intentions were heartfelt: "Sam's only crime was trying to help out a friend. When someone asks for your help, that person is obviously not scared of you or in need of a restraining order."

Judge Aviva K. Bobb continued temporary restraining orders until closing arguments are heard on 21 April.

See Britney & her boys below