Lord Justice Leveson's proposed press watchdog appeared to have run aground last night after David Cameron warned of "serious concerns and misgivings" over legislation.
Speaking in the House of Commons in the aftermath of the report, the Prime Minister broadly welcomed the principles the judge set out to reform the current system.
But he cast doubt on the report's central recommendation that a new system of press self-regulation required a statutory underpinning if it was to command public confidence.
The comments set Mr Cameron on collision course with Deputy Prime Minister Nick Clegg and the leader of the Opposition, Ed Milband, ahead of cross-party talks to agree new regulation.
"I have some serious concerns and misgivings about this recommendation," the Prime Minister told MPs in a Commons statement.
"For the first time we would have crossed the Rubicon, writing elements of press regulation into the law of the land.
"We should, I believe, be wary of any legislation which has the potential to infringe free speech and a free press.
"In this House, which has been a bulwark of democracy for centuries, we should think very, very carefully before crossing this line."
Mr Cameron also expressed concern that legislation would be both highly complex and unnecessary.
Responding, Mr Miliband said Labour endorsed the whole of the "measured, reasonable and proportionate" report and called for legislation to be on the statute book by 2015.
"We on this side unequivocally endorse both the principles set out and the central recommendations," he said – vowing to fight for them to be adopted.
Without statutory underpinning "there cannot be the change we need", he added, calling it the "crucial new guarantee we have never had before", he said.
Mr Miliband added: "The press must be able to hold the powerful, especially us politicians, to account without fear or favour.
"But at the same time I do not want to live in a country where innocent families like the McCanns and the Dowlers can see their lives torn apart simply for the sake of profit."
In an unusual move, Mr Clegg, the Liberal Democrat leader, insisted on making a separate statement to the Commons. The Deputy Prime Minister endorsed Justice Leveson's proposals for press regulator backed by law, and bluntly dismissed Mr Cameron's call for a period of reflection on the controversial plans.
He said: "We mustn't now prevaricate. I – like many people – am impatient for reform.
"And, bluntly, nothing I have seen so far in this debate suggests to me we will find a better solution than the one which has been proposed.
"Nor do I draw any hope from the repeated failure of pure self-regulation that we've seen over the last 60 years. We need to get on with this without delay."
Mr Cameron said he welcomed Lord Justice Leveson's key requirements for a new independent self-regulatory body, including independence of appointments and funding, a standards code, an arbitration service and a speedy complaints handling mechanism.
He said that it should have the power to demand prominent apologies and to impose fines of up to £1 million.
But he said he was in favour of "giving the press a limited period of time in which to do that", but added: "While no one wants to see full statutory legislation, let me state the status quo is not an option."
Downing Street last night insisted said the Prime Minister had not ruled out statutory underpinning but wanted to test whether it was necessary.
"Whether or not there are laws in place, the regulator envisaged by Leveson would be the same," the Prime Minister's spokesman said.
"We think it's difficult to draft legislation in a very narrow and contained way.
"He thinks it would be a very, very big step and we should think very, very carefully before crossing that line."
A senior source said Mr Miliband wanted to proceed on a cross-party basis but added that "the signs are not good".
After the first in a series of cross-party talks this evening, a senior Labour source said that Mr Cameron had agreed to ask the Department for Culture, Media and Sport (DCMS) to draft a bill to implement Leveson's recommendation.
The source said that the talks, lasting less than 30 minutes, were "frank and to the point" and that Mr Cameron's agreement to ask DCMS to do this work came after he was put under pressure by a "robust" Mr Miliband.
According to Labour, the Prime Minister insisted he had not ruled out legislation, while Mr Miliband said that a timetable was needed for the implementation of Leveson's recommendations.
The source said that Labour will move for a vote in the Commons on implementing Leveson in principle by the end of January at the latest.
But a 10 Downing Street source said: "Contrary to Labour's claims, the PM's position has not moved an inch.
"He has deep misgivings about statutory regulation. The exercise of drawing up a Bill will demonstrate how complicated it would be to introduce press laws.
"As the PM said in the House, we have done some similar work over the past few weeks to look at what clauses might look like, and they always end up being more complicated and far reaching than first thought."
Lord Justice Leveson's landmark report also recognised that the online social media world remains beyond regulation, found former Culture Secretary Jeremy Hunt's handling of the controversial BSkyB bid had not been biased and insisted no police briefings should be called "off the record".
The judge said people who found themselves in the public eye for reasons "beyond their control" were treated as "commodities".