Britney Spears' attorney has said that the star lost custody of her children as she was unable to show proof of having a valid driver's licence for California.
The star's lawyer Sorrell Trope told People magazine that doubts over whether the star had taken a drugs test could also have affected the judge's decision to hand over custody of Spears' two children to her ex-husband Kevin Federline.
Trope said: "We weren't able to prove compliance with what the judge ordered.
"Specifically the judge ordered that both parties show valid California driving licenses. I've been unable to produce evidence of that," the lawyer said.
She added that a claim made by the lawyer of Spear's former spouse, that the star had not complied with the judge's directive to appear for drug tests, had also harmed her case.
While stating that there was no evidence to suggest that Spears had failed a drugs test, Trope said the suspicion that she had not followed the order had gone against the singer.
Spears' ex-husband has asked the courts to grant him more time with his children due to his concern for their wellbeing during their time with their mother.
Judge Scott Gordon earlier told Spears that she was a "habitual, frequent and continuous" user of alcohol and controlled substances and ordered her to appear for random drug tests twice a week.
The pop princess has now been issued a temporary California driver's licence after passing a written test.
Spears was recently charged with misdemeanour hit-and-run, for driving without a valid licence. She could face up to six months in jail and be forced to pay a $1,000 (£500) fine if convicted.
Another hearing into the troubled celebrity's custody battle will take place today.
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