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.

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.