Lisa Marie Presley was rushed to the hospital in California on Thursday, her mother said, with US media reporting she was in intensive care following a cardiac arrest.
The 54-year-old daughter of Elvis Presley is in an induced coma on life support, entertainment website TMZ reported, citing unnamed sources.
The outlet said Presley had been "unresponsive" when her housekeeper discovered her at her home Thursday morning in the celebrity-studded Los Angeles suburb of Calabasas.
Her ex-husband, Danny Keough, who also lives at the property, performed CPR until paramedics arrived and transported her to the hospital.
The outlet said its sources stressed it had not been a suicide attempt by the singer.
Priscilla Presley took to Facebook to confirm her daughter was sick.
"My beloved daughter Lisa Marie was rushed to the hospital," she wrote.
"She is now receiving the best care. Please keep her and our family in your prayers.
"We feel the prayers from around the world, and ask for privacy during this time."
The Los Angeles County Fire Department, which runs paramedic services, would not confirm the identity of the patient, but told media that crews had attended a woman in her mid-50s suffering cardiac arrest just after 10:30 am (1830 GMT).
A publicist for Lisa Marie Presley did not immediately respond to an AFP query.
Mother and daughter were seen together on Tuesday at the Golden Globe awards in Beverly Hills, where Austin Butler was awarded Best Actor in a drama for his turn as rock 'n roll legend Elvis Presley in Baz Luhrmann's biopic "Elvis."
Butler, whose fizzing performance has earned wide praise, thanked the two women during his acceptance speech.
"I also want to thank our incredible producers and Warner Bros. and the Presley family. Thank you, guys. Thank you for opening your hearts, your memories, your home to me. Lisa Marie and Priscilla, I love you forever," Butler said.
Lisa Marie Presley was Elvis' only child. She had previously controlled Elvis Presley Enterprises, but sold the bulk of shares in the company to a private equity firm in 2005.
She retained control of Graceland, the Memphis, Tennessee estate her father owned and where he was found unconscious in August 1977.
He was taken to the hospital but pronounced dead from a heart attack. Toxicology reports found high levels of prescription drugs in his system.
Lisa Marie Presley has released three albums this century, and is the mother of actor Riley Keough, who appeared in "Mad Max: Fury Road."
In addition to Danny Keough, whom she divorced in 1994, she has also been married to Nicolas Cage, Michael Jackson and actor/composer Michael Lockwood.