Rupert Murdoch is locked in a secret legal battle against three of his children over the future of the family’s media empire, as he moves to preserve it as a conservative political force after his death, according to a sealed court document obtained by The New York Times.

Mr. Murdoch, 93, set the drama in motion late last year, when he made a surprise move to change the terms of the Murdochs’ irrevocable family trust to ensure that his eldest son and chosen successor, Lachlan, would remain in charge of his vast collection of television networks and newspapers.

The trust currently hands control of the family business to the four oldest children when Mr. Murdoch dies. But he is arguing in court that only by empowering Lachlan to run the company without interference from his more politically moderate siblings can he preserve its conservative editorial bent, and thus protect its commercial value for all his heirs.

Those three siblings — James, Elisabeth and Prudence — were caught completely off-guard by their father’s effort to rewrite what was supposed to be an inviolable trust and have united to stop him. Remarkably, the ensuing battle has been playing out entirely out of public view.

Last month, the Nevada probate commissioner found that Mr. Murdoch could amend the trust if he is able to show he is acting in good faith and for the sole benefit of his heirs, according to a copy of his 48-page decision.

A trial to determine whether Mr. Murdoch is in fact acting in good faith is expected to start in September. Hanging in the balance will be the future of one of the most politically influential media companies in the English-speaking world.