Spears was given permission to choose her own representation when her court-appointed attorney Samuel Ingham III resigned earlier this month.
"As I said in court last week and outside, my firm and I are moving aggressively and expeditiously to file a petition to remove Jamie Spears unless he resigns first," Rosengart said outside of Los Angeles Superior Court, on Monday, July 19.
Rosengart thanked the 39-year-old "for her courage and strength" and her fans for their "support." A group of fans had gathered outside the court house to lend their well wishes.
"The outreach and support for my firm, myself and most importantly Britney has truly been overwhelming — coast to coast and literally throughout the world," he said.
Rosengart's strong words came after Spears' personal conservator, Jodi Montgomery, asked Judge Brenda Penny to charge the conservatorship for extra security that she had to hire after receiving several death threats. The security charges are around $1,800 per day, the Daily Mail reported. The next court date is reportedly scheduled for July 26.
Last week, Spears spoke out about her conservatorship again and said it was "f*****g cruelty" and "if this is not abuse, I don't know what is." The "Toxic" singer also told the court she was not willing to be evaluated in order to remove her father from the conservatorship as she has "serious abandonment issues."
"I would like to charge my father with conservatorship abuse," she said via phone call. "I want to press charges against my father today. I want an investigation into my dad."
Jamie reportedly alleged that Montgomery was behind several of the allegations his daughter made during her initial testimony, which she denied.
"I believe that my client says her wishes are that Ms. Montgomery stay on," Rosengart said during the July 14 hearing, when Spears was asked if she wants Montgomery to remain on her conservatorship team.
Earlier this week on Instagram, Spears said she would not be performing on stage with her father "handling what I wear, say, do, or think" after her at-home dancing videos were criticized.
"My so-called support system hurt me deeply !!!!" the MMC alum wrote, adding, "This conservatorship killed my dreams ... so all I have is hope and hope is the only thing in this world that is very hard to kill."