Los Angeles Superior Court Judge Antonio Barreto Jr. allowed Sizemore to remain free pending an evidence hearing on Nov. 8. But in the meantime, the judge said, the actor's probation "is in a revoked status."
Authorities said Sizemore refused to provide a urine sample for a drug test at a probation office last month.
"He simply walked out," prosecutor Robert Cha said outside court.
Sizemore's attorney said the actor went to his drug counselor the same day and passed a test there. Sizemore declined to take the test at the probation office, attorney Michael Rovell said, because an officer he didn't recognize also wanted to search him there and he feared the results might be tampered with.
"I have every reason to believe him when he tells me that he is clean," Rovell said outside court.
Sizemore, 42, was sentenced in October to six months in jail on misdemeanor charges of harassing, annoying and physically abusing Fleiss, his ex-girlfriend, during their two-year relationship.
Barreto stayed the jail time while Sizemore took part in a rehabilitation program for drug abusers. The judge also said he would consider reducing the sentence to 90 days if Sizemore, who has appeared in such films as "Black Hawk Down" and "Saving Private Ryan," successfully completed the program and other probation requirements.
Prosecutors also recently alleged that Sizemore violated his probation by testing positive for methamphetamine, but his attorney attributed the result to a prescription drug.
Authorities still are deciding whether to file charges against Sizemore in connection with his Aug. 11 arrest at his West Hollywood home, where authorities said they seized narcotics and drug paraphernalia.
The drugs "belonged to somebody else and were planted," his attorney said. He did not elaborate.