The independent directors of News Corp have appointed their own legal team as the company faces shareholder law suits and an investigation by the US authorities.

News Corp's nine independent directors include Rod Eddington, the former chief executive of BA, John Thornton, former president of Goldman Sachs and José María Aznar, the former prime minister of Spain.

Another board member, billionaire investor Tom Perkins, has told the Wall Street Journal the directors have appointed law firm Debevoise & Plimpton, to advise them on the handling of the escalating phone-hacking crisis.

Perkins said: "The board honestly thinks Rupert is a genius and we need him and the company needs him. Our worry is the shareholders at this point. The British police will take care of the hacking victims. The next step is not to let the company go down the drain on this thing because we're focused on events in London that are a small percentage of our business overall."

But the move clearly signals growing tension at News Corp, which already faces one law suit from shareholders who have seen the value of their investment plummet following the revelation of the scandal. It has had a profound impact on the Murdochs' reputation and the company's share price, wiping close to $6bn off its value in the last 10 days.

The move came as Manifest, a leading company advising shareholders on corporate governance, called for Murdoch to step down following his performance at the parliamentary inquiry.

Shares in News Corp rose on rumours that Chase Carey, chief operating officer, was set to succeed Murdoch as chief executive of the company. Murdoch is News Corp's chairman as well as CEO and its largest shareholder. He would remain chairman under the plan, which was first reported by Bloomberg. But Murdoch ruled out any plans to step down, at the hearing saying he was "the best person to clear this up."

At the parliamentary hearing Murdoch pointed out that the scandal occurred at a newspaper that represented just 1% of News Corp's business.

Sarah Wilson, managing director of Manifest, which advises institutional shareholders with £3 trillion in investments, said: "Shareholders may be concerned about Rupert Murdoch's ability to steer an organisation of such immense size and complexity. There are succession issues to be addressed."

Manifest's statement follows a call from corporate governance group PIRC for James Murdoch, Murdoch's heir apparent, to step down as chairman of BSkyB. "Other investors share our opinion," it said in a statement. "The risk of contagion is great. … How, in reality, can someone facing challenges on all sides expect to devote sufficient attention to chairing a FTSE 100?" On Monday, credit agency Standard & Poor's put News Corp's credit rating on watch, citing the risks associated with widening legal investigations in Britain and the US. Rich Greenfield, media analyst at BTIG, said that the Murdochs' control of News Corp had long been a handicap to the company's share price. "The stock trades at a discount to its peers," he said. He said the company's dual class share structure, in which the sahres conrolled by Murdoch have greater voting rights, had long been off putting to investors. But he said that there was no evidence that the scandal was affecting News Corp's other assets. "People are still watching Fox and going to see their movies," he said.

Claire Enders, founder and chief executive of media researcher Enders Analysis, said: "Clearly there are a lot of shareholders who are freaked out. It's hard not to see US shareholders looking at the Murdochs and seeing them as responsible." She said:

"There are now credibility issues that will hang over the Murdochs come what may."

The UK phone hacking scandal is now being investigated by the US authorities