Media giant News Corp is mustering its forces to make sure the Murdoch family stays in control and to ward off a possible takeover bid.
It is planning a "poison pill" defence, allowing shareholders to increase their stakes if anyone buys more than 15% of the company.
The 73-year-old Rupert Murdoch and his family own 29.5% of the firm.
The plan comes days after US cable group Liberty Media moved to raise its holding from 9% to 17%.
Liberty boss John Malone has, to date, been a long-time ally of Mr Murdoch.
The move could be a prelude to a full-scale bid, or alternatively to merge the two empires.
News Corp said it had not been told in advance about Mr Malone's move.
"It's too early to tell what Liberty's intentions are, but we're not necessarily treating them as friendly," a spokesman said.
Protection
If implemented, the poison pill plan gives existing shareholders the right to double their holdings at half the current share price if anyone takes control of 15% of the company's shares.
The rule does not apply to Mr Malone just yet.
Liberty's existing 9.1% stake and its option on another 8% currently held by investment bank Merrill Lynch are both declared, exempting them from the plan.
But if Liberty were to buy more than a further 1% of News Corp's voting shares, all other shareholders would immediately be allowed to expand their stake.
The effect would be massively to dilute Liberty's - or another potential bidder's - holding.
News Corp has been based in Australia since its inception, but is shortly to move its headquarters and its incorporation to the US from Australia.
Mr Murdoch himself switched his citizenship to the US in 1985, so as to comply with US rules banning foreigners from owning US broadcasters.
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