Harriet Harman has suggested a 15 per cent ownership cap across newspapers, broadcasting companies and online sites to stop companies from feeling like they are above the law.
The shadow culture secretary has also suggested that there should be a robust 'fit and proper person' test, which would “cover impropriety and failures of good governance, and allow for interventions in a merger on propriety as well as plurality grounds."
Speaking at the annual Charles Wheeler Lecture on journalism later today, hosted by the University of Westminster and the British Journalism Review, Harman is expected to say: "The Leveson Inquiry focused on the complaints system and the impunity which came from the lack of a robust, effective complaints system was undoubtedly a key part of the problem.
"But so too was something else in Leveson's terms of reference which he was not able take forward in such depth. The invincibility that came with too much power concentrated in the hands of one man.
"Media monopoly matters in a democracy. The concentration of unaccountable media power distorts the political system. The media shapes how we see ourselves and how we see the world. In a democracy, the free flow of information, of different points of view, is crucial for open debate.
"Too much power in too few hands hinders proper debate. Plurality ensures that no media owner can exert such a damaging influence on public opinion and on policy makers. It ensures that no media company can have so much influence that it feels itself immune, above the rule of law. It ensures no private interest can set itself above the public interest.
"But we don't have a proper regime for protecting against this. The system doesn't work - its inadequacies and complexities were laid bare by the News Corp bid for the whole of BSkyB. And the system is out of date - this is an age of great change in the media, where we have print newspapers, broadcast media and new media, and a convergence of all three.
"Now is the time for reform, to tackle this problem that has persisted for many years. We feel it would be best for the government to go about this in a cross-party way."
At Leveson, Labour suggested a 30 per cent ownership cap within the newspaper industry.