Ed Miliband has said his decision to call for Rebekah Brooks to resign from News International would have been seen as an "act of war" by Rupert Murdoch's company.
Giving evidence to the Leveson inquiry on Tuesday, the Labour leader said his decision on July 5 to call for an inquiry into phone hacking and for Brooks to quit on 5 July 2011 was "crossing a Rubicon".
"I knew it would be seen by News International as an act of war," he said.
"There should be no interest too powerful in this country, in banking or in the press or anywhere, that politicians don't speak out about if they think there is wrong doing," Miliband said on Tuesday.
"That is the job of democracy, it's to speak out, that's what the people elect us for."
Brooks eventually quit her job as News International chief executive on July 15, eight days after the News of the World was shut down.
However Miliband, who served in Gordon Brown's cabinet, admitted he should have spoken out sooner than he did.
"There is clearly something which has gone very wrong with the way parts of the press dealt with individuals," Miliband told Lord Justice Leveson.
"A failure to get to grips with these issues ... by the press, the police, who did not investigate properly, and I think politicians, who were aware of some of what was going on and did not speak out."
He added: "Organisations like News International had huge power and I think politicians were reticent to speak about some of these practices that were exposed. I include myself in that."
Miliband told Leveson that he would "do everything I can to seek on cross-party basis to ensure your recommendations provide a framework for the future".