Ed Miliband has urged MPs to "stand up for the victims" of press abuse by enshrining a new press regulator in law in a crunch Commons vote.
"Monday is the day that politics has got to do the duty by the victims and has got to stand up for the victims," he told The Observer.
"I think it is an important moment because we have had decades of failing to ensure that we have a system of press complaints and redress which means that ordinary people aren't left at the whim of a sometimes abusive press."
Rival proposals for a new system of newspaper self-regulation will face judgment on Monday in a high-stakes battle over the response to the phone hacking scandal. In a reversal of the Westminster status quo, Labour and the Liberal Democrats are lining up against the Conservatives in a coalition for a tougher regime.
Both sides propose using a royal charter to create a new watchdog body in response to the recommendations of the Leveson Inquiry into abuses by the press. But the Lib/Lab version involves underpinning the system in legislation, something which has been opposed by Prime Minister David Cameron as a risk to press freedom. It also rules out a veto for the press over the membership of the watchdog and gives the regulator the power to enforce the publication of prominent apologies.
Mr Cameron - who dramatically pulled the plug on cross-party talks on Thursday - has said he will abide by the will of Parliament. Without an overall Commons majority, allies accept he is unlikely to win sufficient support for using the threat of punitive damages against non-participants to persuade papers to sign up. Despite efforts to shore up the vote - including bringing a minister back from Japan - a number of Tories are expected to back the Lib/Lab proposals.
Mr Cameron insists his proposal is "the fastest possible way to deliver the strong self-regulation body that Leveson proposed that can put in place million-pound fines, prominent apologies and get justice for victims in this country".
But Mr Miliband said politicians of all colours now needed to follow their instincts about what was right. He told The Observer: "Now we are at this moment which is a sort of crossroads: do we change or is it more of the same? I think we need to choose the right course, and I think it is a test of politics."
There was criticism of the failure to incorporate fully Lord Justice Leveson's proposals from the parents of murdered schoolgirl Milly Dowler. It was the revelation that News of the World journalists had hacked Milly's phone which prompted the paper's closure and the 16-month Leveson Inquiry.
Bob and Sally Dowler said: "Given the considerable investment of time and money in the Leveson Inquiry, we are very disappointed to learn that Lord Justice Leveson's proposals may not now be taken forward if the politicians choose to ignore the recommendations of Lord Justice Leveson that were aimed at preventing the sort of abuses that we and so many others suffered," they said.