Nevada commissioner Edmund Gorman concluded in a decision filed on Saturday that Rupert Murdoch and Lachlan, who is the head of Fox News parent Fox Corp and News Corp, had acted in "bad faith" in their effort to amend the irrevocable trust, the Times reported.
The court docket indicates it issued a recommendation or order Saturday under seal.
The trust currently would divide control of the company equally among Rupert Murdoch's four oldest children - Lachlan, James, Elisabeth and Prudence - after his death.
Potentially, three of the heirs could out-vote a fourth, setting up a battle over the future of the companies, even as Lachlan Murdoch runs Fox and is sole chair of News Corp.
A spokesman for Rupert Murdoch, 93, could not immediately be reached for comment.
Rupert Murdoch's proposed amendment would have blocked any interference by three of Lachlan's siblings, who are more politically moderate.
In his opinion, Gorman said the plan to change the trust was a "carefully crafted charade" to "permanently cement Lachlan Murdoch's executive roles" inside the empire "regardless of the impacts such control would have over the companies or the beneficiaries" of the family trust, the Times said.
A lawyer for Rupert Murdoch, Adam Streisand, said they were disappointed with the ruling and intended to appeal, the Times reported.
