He addressed reporters outside of the Los Angeles County Courthouse on Monday, and reiterated his statements from Britney's conservatorship hearing last week.
"My firm and I are moving aggressively and expeditiously to file a petition to remove Jamie Spears," Rosengart stated, "Unless he resigns first."
Rosengart also clarified that Monday's hearing was "about was an issue concerning security." Recently, Britney's personal conservator, Jodi Montgomery, filed a request for increased security -- a request Jamie has opposed and objected to.
Rosengart did, however, share his thanks for the overwhelming support that has been expressed by Britney's outspoken and dedicated fans.
"I once again want to thank Britney Spears for her courage and for her strength," Rosengart shared. "I want to thank judge Penny for her courtesy in welcoming my firm and I into this case. And I also want to thank Britney Spears' fans and supporters."
Rosengart added, "The outreach and support for my firm, myself and most importantly Britney has truly been overwhelming. From coast to coast and literally throughout the world."
Britney originally requested the right to hire her own attorney during her testimony at a hearing on June 23, at which point she was represented by her court-appointed attorney of 13 years, Samuel Ingham, who subsequently resigned.
"Britney Spears finally gets to hire her own attorney, her own advocate to fight for her. She gets to choose someone, she gets to vet the background, and she gets to make that decision," said Stockman, president of the Law&Crime Network. "I think it's a big move and I think it's a really important one."
During the hearing on July 14, Britney also reiterated several of her claims of abuse against her father, and alleged that she's "always been extremely scared" of Jamie because she claims she "thought he was going to show up drunk."
She repeated her claims that the conservatorship felt like "f**king cruelty" and said of those in charge of her conservatorship, "I thought they were trying to kill me."
"I want to press charges against my father for conservatorship abuse," Britney said, adding, "I want an investigation into my dad."
To this point, Rosengart stated during the hearing that his firm "will be moving promptly and aggressively for [Jamie's] removal."
"The question remains, why is he involved? He should step down voluntarily because that’s what’s in the best interest of the conservatee,” Rosengart added.
For more on the pop icon's ongoing conservatorship case, see the video below.