Rupert Murdoch has told employees at Fox Corp and News Corp that the companies were “in robust health, as am I” as he announced he was stepping down as the firms’ chairman.

“Our opportunities far exceed our commercial challenges,” he said in a letter seen by Reuters.

“We have every reason to be optimistic about the coming years – I certainly am, and plan to be here to participate in them. But the battle for the freedom of speech and, ultimately, the freedom of thought, has never been more intense.”