Jamie Spears, 68, made the claim on forms that he used to warrant having power over his daughter’s finances.
Court documents obtained by The Battle for Britney documentary-makers show the reason for Ms Spears’ conservatorship, which was enforced in February 2008.
Journalist Mobeen Azhar said Mr Spears had ticked a box saying it related to “dementia placement or treatment”.
Mr Azhar added: “There’s only two options with that. Britney might have dementia. Now I’m not a doctor but if that’s the case, then the world isn’t aware of that. But the other option is actually more sinister.
“That’s the idea that she doesn’t have dementia but the conservatorship team have suggested that she has because they want to push the conservatorship through. And if that’s the case, then that’s terrifying.”
Free Britney campaigner Megan Radford says the claim meant she could be placed in a probate conservatorship, rather than a mental health one which nobody can earn money from.
She said Britney “clearly did not and does not have dementia”, and called her father’s case “fraudulent”.
The conservatorship was put in place in 2008 following a year in which Britney divorced backing dancer Kevin Federline, 43, shaved her head, attacked a photographer’s car with an umbrella and entered rehab.
She also lost custody of sons Sean, now 15, and Jayden, 14, and was committed to a psychiatric hospital.
Mr Spears retains joint control of her estate alongside a wealth management company, despite Ms Spears asking for him to be removed from his role.
His conservatorship means the Toxic singer cannot make any financial or professional decisions without her father's approval.
Judge Brenda Penny, sitting at Los Angeles Superior Court, set a status hearing for June 23.