According to The Washington Post, Marion Jones had admitted to using steroids before the 2000 Olympics.
The report says the track and field star admitted to steroid use in a recent letter to close family and friends.
According to the report, Jones says she'll also plead guilty tomorrow to two counts of lying to federal agents about her drug use and an unrelated financial matter.
Jones, who won five medals at the Sydney Games, reportedly says she took "the clear" for two years, beginning in 1999, and that she got it from former coach Trevor Graham, who told her it was flaxseed oil.
"The clear" is a performance-enhancing drug linked to BALCO, the lab at the center of a federal steroids investigation.
Until now, Jones has steadfastly denied she ever took any kind of performance enhancing drugs.