Los Angeles superior court commissioner Scott Gordon said Spears was a "habitual, frequent and continuous" user of alcohol and controlled substances and ordered the star to attend bi-weekly drug tests that would be conducted at random dates.
The Baby One More Time star has also been directed to enrol in one-to-one counselling sessions on childcare issues and to meet with a parenting coach every week, who will assess the singer's progress.
The judge also ordered both Federline and Spears to refrain from certain behaviour, such as making derogatory statements about one another and using corporal punishment on their children.
Both Spears and Federline must also complete a programme called Parenting Without Conflict and attend joint co-parenting counselling.
In addition to the meetings with childcare experts, the judge said that neither party must consume alcohol or drugs 12 hours before meeting their children. The 50-50 custody arrangement for the couple's two children remains unchanged.
The news is another blow to the star after her manager and lawyer ended their association with her yesterday.
Spears, whose latest album will be released in November, was universally ridiculed for her performance at the MTV video music awards. The singer was criticised for a "zombie-like" and lacklustre performance of her single Gimme More at the show.
