Jamie, 66, sued the creator of the Absolute Britney blog for defamation, the Associated Press reported Wednesday evening.
The blog is devoted to the conspiracy theory that Britney, 37, is being controlled against her will under Jamie's conservatorship, which began in 2008 after the "Baby One More Time" singer's highly publicized breakdown.
The lawsuit, filed in Los Angeles, claims that Absolute Britney website creator, Anthony Elia, made false and malicious claims that Jamie and his conservatorship controlled Britney's Instagram account to make her seem less stable and more in need of psychiatric help than she actually is.
"It is time for the conspiracy theories about Britney Spears' well-being and the mob #FreeBritney movement to stop," the lawsuit says in its opening line.
Elia's blog and its social media accounts have reportedly greatly influenced the "Free Britney" phenomenon, a group of fans who say Jamie is controlling Britney against her will, including through her social media accounts, and keeping her from freeing herself of the conservatorship despite apparent progress she's made over the last decade.
Jamie's suit alleges that Elia "has made it his mission to spread false and malicious lies on the internet about Britney, her conservatorship and her team, including that those around Britney are harming her and not acting in her best interests."
Elia has not responded to requests for comment.
The suit against Elia comes two weeks after a judge granted Britney and the conservatorship a five-year restraining order against Britney's former manager Sam Lutfi, who worked for the singer leading up to her public meltdown. It prohibits him from trying to undermine the conservatorship by contacting members of the Spears family or writing disparaging posts on social media about Britney, her family or her case.
A conservatorship, known in some states as a guardianship, is normally reserved for those who are severely debilitated. One was established for Spears 11 years ago during her very public meltdown, and has been kept in place ever since. The arrangement has increasingly come under attack from fans, who have grown especially loud about it in recent months.
Britney has not commented publicly on her conservatorship, but did request to speak to the judge in her case at a closed hearing in May. The judge subsequently ordered a court review of her situation before another hearing, currently scheduled for September.
The suit focuses on a June 14 Absolute Britney Instagram post that alleges the conservatorship was seeking to manipulate Britney Spears' Instagram account.
"Britney's team is deleting positive comments on her Instagram post and leaving negative ones to keep up the illusion that she needs help!" the post says. "This has to be a human rights violation!!!!"
The post, which the suit calls "false and defamatory," was discussed by bigger blogs and some media outlets. Elia shared some of those reports. Britney's mother, Lynne Spears, stoked the rumors by responding to similar comments to Elia's on Britney's account.
Jamie's lawsuit seeks unspecified damages and a court injunction permanently prohibiting Elia from spreading information on the conservatorship.
In April, she announced in an Instagram post that she was taking time off to focus on self-care. She reportedly checked into a mental health treatment facility the same month.
Since leaving treatment, Britney has been working out furiously following the publication of paparazzi photos she deemed unflattering. She most recently enjoyed beach vacations with longtime boyfriend Sam Asghari in Miami and mom Lynne in Turks and Caicos.
The Associated Press contributed to this report.