How Britney Spears and Kevin Federline Went From Friendly Exes to Feuding (Exclusive)

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A source tells ET that the former couple were friendly exes, until Federline starting asking for more child support.

Things are getting complicated between Britney Spears and Kevin Federline. 

The former couple has been engulfed in a child support dispute over the last few months, with things now reaching a new point as a source tells ET that Spears was served with a subpoena to sit down for a deposition with Federline's attorney, Mark Vincent Kaplan. 

According to ET's source, Spears' attorney, Laura Wasser, accepted the subpoena, which also calls for the singer's most recent tax returns and an income and expense declaration, on her behalf. The GRAMMY winner has been on vacation in Miami with her and Federline's sons, 12-year-old Sean Preston and 11-year-old Jayden James. 

Federline has had full custody of their sons since 2008, and according to court docs obtained by ET, he currently receives $20,000 a month to take care of them. Kaplan told ET in March that Federline is seeking at least double the child support he currently receives. A second source tells ET that Spears spending more time with her sons might be a reason behind Federline's request for more child support, and his and Spears' souring relationship. 

"Britney and Kevin had a cordial relationship until Kevin started asking for more money. Now they only speak through attorneys," the source claims. "Last summer, Britney offered to take the boys more often and even discussed having more custody. That seems to be what started this entire ordeal."

"Some of Kevin's friends think he feels threatened because Britney is doing so well that she might not need him to have the kids as much anymore. Therefore, his child support might plummet," the source continues. "Kevin needs that money to keep up the lifestyle that he has become accustomed to."

"Things have changed over the last year. Britney and Kevin used to attend the same baseball games and birthday parties together but their relationship has been damaged because of Kevin's demands for increase in his current child support," the source says. "Kevin is claiming there is far too much of a discrepancy between how they live at his home compared to Britney's."

While ET previously learned that Federline and Spears first attempted to work out their expenses privately, Federline filed court docs in Los Angeles asking for an increase in child support last month based on Spears' rising income due to her Las Vegas residency and worldwide tour, reported by Forbes, and his lack of steady work as a DJ. According to ET's second source, Spears is "getting a full forensic report on Kevin to try to prove where the money she provides is actually being spent." In addition to his two kids with Spears, the former backup dancer is also father to four other children. 

"Britney is truly on track. She looks great, and feels great and loves being with her kids. She has come such a long way since she and Kevin first split," the source shares, adding that despite their parents' battle in court, Sean and Jayden are doing well. "[They] are truly well behaved and happy, good kids, thanks to the support of so many family members."

Reporting by Adriane Schwartz.