David Cameron has been told by Harriet Harman to "man up" and deal with press regulation.
Labour's deputy leader said her party wanted to reach agreement on the issue that is dividing the coalition, but not if the Tories "drive a coach and horses" through Lord Justice Leveson's report.
The political parties are at loggerheads over how to implement press reform in light of the phone hacking scandal.
The Conservatives have put forward plans for a 'royal charter' to manage press regulation, similar to the arrangement with the BBC.
But Labour and the Liberal Democrats want to see Leveson, which called for statute to back up a new regulator, implemented in full.
In an interview with the BBC's Andrew Marr programme, Harman said the opposition would "definitely look at a royal charter" but not if the judge's recommendations had been "watered down".
She claimed Labour had "bent over backwards" and hoped the Prime Minister would not be "lent on" by newspaper who are vehemently opposed to state involvement.
In a direct challenge to the Prime Minister, she said: "I think it's now time for him to, you know, man up, step forward and actually say 'yes, we are going to do it' and then we will agree with him and we will support it."
The Tories, who are eager to avoid using new laws to deal with the issue, published their proposals on February 12, and were immediately accused by Labour of 'diluting Leveson'.
So divisive were Leveson's proposals that Nick Clegg and Cameron took the rare step of making conflicting statements to MPs when they were published in November.
Harman claimed MPs were growing impatient for action after the House of Lords voted in favour of using statute.
She added: "So really it's for the Conservatives now to really take this historic opportunity to sort out what has been wrong for decades and we'll support them if they do that."