Kevin Federline Alleges Britney Spears Did Cocaine During Time She Was Breastfeeding

Kevin Federline has dropped another bombshell Britney's way ... claiming she did cocaine during the same time she was breastfeeding their kids.

In an excerpt from his new tell-all, "You Thought You Knew," obtained by Variety, Federline recalls walking in on her, witnessing her doing coke and getting into a huge fight that led to their divorce.

He claims she was supposed to be home with their sons Sean Preston and Jayden James, but showed up to his 2006 release party for "Playing With Fire."

“As we reached the top [of the venue], Jamie Spears, Britney’s dad, was standing there,” Federline writes. “I hadn’t expected to see him, so I was happy he’d decided to come support me. He threw a look my way, shook his head and motioned to my dressing room door. His expression sat in that flat blank space between disapproval and apprehension, as if to say: ‘She’s here, and it’s not good.'”

He continues, “The first thing I saw was Britney and her young starlet friend snorting a fat line of coke off the table,” Federline alleges. “Both were wearing these outrageous wigs. Britney’s was electric blue. It was surreal. They didn’t even try to hide it.”

K-Fed was on "TMZ Live" Wednesday and talked about other issues in their marriage, including a secret call she made to her ex, Justin Timberlake, the night before the nuptials.

As for the night he allegedly caught Brit doing coke ... in the book he says he begged her not to "go home and breastfeed the kids like this. Call your mom or someone. We need to get formula. You can't do this."