The elder Spears alleges in "Through the Storm: A Real Story of Fame and Family in a Tabloid World" that the singer's former manager, Sam Lufti, threatened her daughter several days after the "Toxic" star's visit to a psychiatric ward in January, the New York Post reported Sunday.
"If you try to get rid of me, she'll be dead and I'll (expletive) on her grave," Spears quotes Lufti as saying.
"Tabloid World" also includes allegations that Lufti would keep Spears drugged by slipping sleeping pills into the pop star's food and drink prior to the January stay.
The elder Spears said before her family obtained a restraining order against Lufti, he allegedly had complete control over her daughter's life and would prevent them from meeting, the Post reported.
"Within a month of meeting, Sam was in complete control of (Britney's) life. He appointed himself as her gatekeeper, and there was no one he wanted to keep the gate closed to more than Britney's family," Lynne Spears alleged.