McNew/GettyLindsay Lohan failed a court-mandated drug test last week, a source told the Daily News

Lohan gives a smirk in her booking photo at the Century Regional Detention Facility in Lynwood, Calif.

LOS ANGELES - Jail and rehab couldn't scare Lindsay Lohan straight - she's flunked a drug test, a source told the Daily News Friday.

The positive result on a court-mandated test last week could send the "Mean Girls" starlet back to the slammer.

TMZ.com reported the test was for cocaine.

Prosecutors declined comment, and Lohan's lawyer could not be reached.

Lohan, 24, denied it at first. "They're all nuts," she told Us magazine. "I'm fine."

But the troubled starlet later copped to the failed drug test Friday night through several Tweets saying she's "prepared to appear before Judge Fox next week as a result."

"Substance abuse is a disease, which unfortunately doesn't go away over night. I am working hard to overcome it and am taking positive steps forward every day. I am testing every single day and doing what I must do to prevent any mishaps in the future," Lohan said in another tweet to her 1.18 million followers.

LiLo is on probation until next August for back-to-back 2007 DUI cases.

She was sentenced to 90 days in jail this summer for missing booze counseling classes, but served only 14 due to overcrowding.

The court ordered a 90-day inpatient rehab stint, but doctors said she could leave after 23 days.

The judge put her on a strict post-jail program that includes counseling and random testing until Nov. 1.

"It appears she is very serious about her sobriety," Judge Elden Fox said at the time.

"She is very serious," Lohan's lawyer Shawn Chapman Holley agreed. "She has learned her lesson and wants to move on in a positive way."

The judge also issued a stern warning: "Any positive or missed random tests are to be reported to the court in 72 hours and result in a violation of probation with 30 days in county jail."

Because she was convicted of three counts, that could mean up to 90 days in lockup if she's found to have violated probation.

Read more: http://www.nydailynews.com/gossip/2010/09/17/2010-09-17_lindsay_lohan_fails_courtmandated_drug_test_could_be_sentenced_to_30_more_days_i.html#ixzz0zsNtAg5f