Britney Spears is out to prove her ex is a deadbeat dad with a marijuana problem as her divorce battle hits a new low, according to a new U.S. report. The troubled pop star has reportedly accused Kevin Federline of smoking pot around the couple's sons - to counter claims she boozes in front of the kids. Spears lost physical custody of her sons in October (07) after failing to follow court-imposed drug and alcohol testing, but now she's firing back with allegations Federline exposes the couple's kids to marijuana. A source tells American magazine Life + Style, "Britney believes Kevin's been smoking pot in front of the kids. She says she can smell it in the boys' hair and on their clothes when she has her visitation with them." And the publication claims Britney's so upset, she's determined to have Sean Preston, two, and Jayden James, one, drug-tested to confirm her suspicions. The source adds, "She's asked the court-appointed monitor and her lawyers if they can prove the kids have been exposed to environments in which drugs have been taken." But a pal of Federline's, who has rapped about his love of marijuana, insists Spears' ex-husband "never smokes (pot) around the kids".