Ed Miliband called today for an overhaul of media ownership rules as he warned that Rupert Murdoch's influence on British politics was "dangerous".
In a fresh salvo against Mr Murdoch's under-fire News Corp, the Labour leader said he wanted to forge a cross-party agreement on plans that would reduce the media mogul's UK market share.
In an interview with The Observer, he said: "I think he has too much power over British public life."
The News Corp empire has been diminished with the closure of the News of the World over the phone hacking scandal, but it still owns The Sun, The Times, The Sunday Times and 39% of BSkyB.
Having already called successfully for the resignation of News International chief executive Rebekah Brooks, Mr Miliband said the Government now needed to look at media ownership rules.
"I think that we've got to look at the situation whereby one person can own more than 20% of the newspaper market, the Sky platform and Sky News," he said.
"I think it's unhealthy because that amount of power in one person's hands has clearly led to abuses of power within his organisation.
"If you want to minimise the abuses of power then that kind of concentration of power is frankly quite dangerous."
Meanwhile, the Liberal Democrats have written to media regulator Ofcom urging it to consider whether the owners of the BSkyB licence are "fit and proper" given the allegations swirling around News Corp.
Lib Dem deputy leader Simon Hughes, media spokesman Don Foster and party president Tim Farron asked the watchdog to investigate in light of "the manifest public concern about News International's activities, the close integration of News International with its parent company News Corporation, (and) News Corp's effective control of BSkyB".
Ofcom said it would consider its response to the Lib Dem letter next week.
"We have received a number of inquiries about Ofcom's ongoing duty to be satisfied that broadcasters remain fit and proper to hold Broadcasting Act licences," a spokeswoman said.
"In considering 'fit and proper', Ofcom will take into account any relevant conduct of those who manage and control such licences.
"As we set out in our letter to the Chair of the Culture, Media and Sport Select Committee last week, we continue to gather information which may assist us in the discharge of our duties.
"We have already written to a number of relevant authorities and can confirm that follow up meetings will now be taking place."
Deputy Prime Minister Nick Clegg said he also wanted greater "plurality" in the media, although he suggested that cross-party talks on the issue should wait for the recommendations of the judge-led inquiry set up in response to the hacking scandal.
"We do need to look again in the round at the plurality rules to make sure there is proper plurality in the British press," he told BBC1's The Andrew Marr Show.
"A healthy press is a diverse one, where you've got lots of different organisations competing, and that's exactly what we need."
He said the Lib Dems had been calling for the change for years but said he was "very happy to sit down" with Mr Miliband.
"The judge-led inquiry will of course during the course of a year produce some ideas about what we should do, and then I think if we can act on it on a cross-party basis as we did last week in the House of Commons, all the better."
Defence Secretary Liam Fox appeared to warn Mr Miliband against "jumping on bandwagons and getting today's headlines".
And he spoke of politicians wanting to "get their own back on some elements of the media".
The Tory Cabinet minister told Sky News' Murnaghan: "Obviously any government is going to continually want to look at how the Press is structured, but we do have a very pluralistic Press in this country.
"I think politicians would be wise at the moment not to over-react, that there is a definite feeling of politicians wanting to, if you like, get their own back on some elements of the media.
"It's time for calmness and for people to look at these issues, yes seriously, but in a proper timescale and without hysteria."
Further regulation of the Press should only happen if it was "absolutely necessary", he added.
"I think we have to be proportionate about how we deal with any of these issues, it can't be simply about jumping on bandwagons and getting today's headlines.
"It has to be a serious look at how we deal with the wrongdoings that have taken place already and how we go about future regulation of the Press.
"Having a free Press is something that Western democracies do extremely well, it's something we should treasure and we should only be looking at further regulation if we believe it to be absolutely necessary."
Asked whether Mr Murdoch's contribution to British media had been positive, Dr Fox spoke of the mogul's efforts in "defeating the print unions at Wapping through to building up a profitable business and making the printed Press more profitable, not only for News International but other newspapers as well."
But he said this had to be balanced against the "very clear" wrongdoings at the News of the World.
Labour's deputy leader Harriet Harman said Mr Murdoch's media empire was "too powerful and too rich".
The "concentration of power" that he had built up should not be allowed to happen again, she said.
"It was too mighty for government, it was too mighty for the police, and actually it was Ed Miliband and parliament that has stopped it in its tracks," Ms Harman told Sky News' Murnaghan programme.
She added: "News International was too powerful and too rich and has had a malign influence, and that's why Ed Miliband is saying that empire should be broken up and we should have a free and fair press."