The actor suffered a "severe anoxic brain injury," caused by a lack of oxygen, when her car crashed into a Los Angeles area home on August 5 and fire erupted, according to a statement released Thursday that said she wasn't expected to survive.
She's been hospitalised at a Los Angeles burn centre.
"This is a sad day. I'm sending Anne's children, family and friends all of my love," Ellen DeGeneres said Friday on Twitter amid reports of Heche's dire condition. They were a couple from 1997 to 2000.
Detectives investigating the crash said narcotics were found in a blood sample taken from Heche, though toxicology tests still had to be performed to differentiate them from drugs she was given for her injuries, Los Angeles police said.
A native of Ohio, Heche first came to prominence on the NBC soap opera "Another World" from 1987 to 1991. She won a Daytime Emmy Award for the role of twins Marley and Vicky.
In the late 1990s she became one of the hottest actors in Hollywood, a constant on magazine covers and in big-budget films. In 1997 alone, she played opposite Johnny Depp as his wife in "Donnie Brasco" and Tommy Lee Jones in "Volcano" and was part of the ensemble cast in the original "I Know What You Did Last Summer."
The following year, she starred with Harrison Ford in "Six Days, Seven Nights" and appeared with Vince Vaughn and Joaquin Phoenix in "Return to Paradise."
- AP