Rupert Murdoch has moved to hand control of his family trust to his eldest son, Lachlan, sparking a bitter legal war between his children that could determine the direction of two of the world’s most powerful media businesses.

The 93-year-old media baron has sought to overhaul an irrevocable family trust to give Lachlan full control of its voting powers and decision-making after the patriarch’s death, according to three people familiar with the situation. Lachlan had already assumed the leadership of Fox and News Corp last year when his father stood down as chair of the businesses behind Fox News, The Times and The Australian.

The billionaire news mogul has tried to enshrine this move by amending the trust that holds the family’s interests in the two companies, these people said. Control of the entity was supposed to be split between his four eldest children — Lachlan, James, Elisabeth and Prudence — when he dies.