Rupert Murdoch's efforts to entrench his son Lachlan as primary heir to his media empire have been dealt a legal blow, writes Matthew Cox.

Nevada's commissioner has ruled that an attempted amendment to the family's trust was made in "bad faith".

The trust, established in 2006, divides control of Fox News and News Corp, which owns The Sun and The Times, equally between Mr Murdoch's four eldest children in the event of his death.

The proposed amendment would not change the division of company shares between Lachlan, James, Prudence and Elisabeth, but was about the control of the multibillion-dollar media empire.

There is a clause that allows Mr Murdoch to amend it before his death so long as it was in the interests of his beneficiaries.

He and Lachlan sought to prove in sealed testimony in Reno in September that the latter's more Right-wing stance would better ensure the financial security of the businesses.

However, Mr Gorman, in a sealed document seen by the New York Times, was not swayed.

The 93-year-old media mogul intends to appeal against the decision.

Mr Gorman's ruling still needs to be ratified by the Probate Court.

Photo: Rupert Murdoch, co-chairman of Twenty-First Century Fox Inc., left, and Lachlan Murdoch, co-chairman of Twenty-First Century Fox Inc.. Photographer: David Paul Morris/Bloomberg via Getty Images