David Cameron must back Lord Justice Leveson's proposal for a new law to regulate the press by Christmas, Labour leader Ed Miliband has said.
If the Prime Minister has not signed up by then, Mr Miliband said he will pull out of cross-party talks.
In an interview with The Observer, Mr Miliband said the Tory leader has "one last chance to show leadership" or his party will begin rallying for a Commons vote in January, which could see Mr Cameron defeated.
He said: "We're not going to let these talks become a smoke screen for inaction and just be used as a way to run this into the ground, hoping people forget all about it, and hoping the fuss dies down.
"So in the next two to three weeks we have got to have a resolution.
"If the cross-party talks have nowhere to go and we can't reach agreement we have to go to parliament."
Mr Miliband backs Lord Justice Leveson's report, published on Thursday, agreeing that self-regulation of the press needs to be backed by law, because it has so far "failed".
He said the "ingenious and good" solution is similar to the Irish model but with greater political distance.
David Cameron betrayed the promise he made to victims and has failed in his leadership by not standing up for them, he added.
But the opposition leader called Deputy Prime Minister Nick Clegg "courageous and right" on the issue.
Earlier this week Mr Clegg pledged to make sure legislation implementing the press regulation plan was drawn up "in good faith".
Cross party discussions have already begun, but Mr Miliband said by Christmas it will be clear if progress can be made or whether Mr Cameron wants to "kick things into the long grass".