Christopher Federline made the allegation in court papers filed for a temporary restraining order against the 31-year-old pop star, Radar Online reported.
In the papers filed in U.S. District Court in Tampa, Florida, Federline alleged that the X Factor judge blackmailed and harassed him and stole away his credit card withdrawing 4,500 dollars from his bank account.
He added that after he confronted her about the theft, Spears allegedly made fun of him and told him he had a "small penis".
Christopher also claimed in court docs that Spears blackmailed him that if he tells the police about the credit card, she will tell the world that he's the true father of her now-7-year-old son Sean Preston, not Kevin.
He said that in the recent years, Spears had used their secret to gain leverage over him.