Labour leader Ed Miliband has called for new media ownership rules to limit Rupert Murdoch's "dangerous" and "unhealthy" concentration of power.
He told the Observer Mr Murdoch's large market share led to "abuses of power".
Deputy Prime Minister Nick Clegg backed new ownership rules to foster more press diversity but said an independent inquiry should be completed first.
The calls follow last week's closure of the News of the World, which Mr Murdoch owned, amid claims of phone hacking.
With that closure, the Sun, the Times, the Sunday Times and 39% of digital broadcaster BSkyB remain in the News Corporation stable.
Under pressure from the entire British political establishment, Mr Murdoch dropped plans to buy out the rest of British Sky Broadcasting.
Calling for new ownership rules, Mr Miliband said: "I think that we've got to look at the situation whereby one per can own more than 20% of the newspaper market, the Sky platform and Sky News.
"I think it's unhealthy because that amount of power in one per's hands has clearly led to abuses of power within his organisation