Labour has accused David Cameron of "an historic mistake" after he called off talks on press reform.
Deputy Prime Minister Nick Clegg joined the criticism, saying he was "disappointed" and "surprised" at the Tory leader's decision.
They were speaking after the Prime Minister called off cross party talks aimed at thrashing out a agreement over Lord Justice Leveson's report, set up in the aftermath of the phone-hacking scandal.
Even a senior Conservative, former Cabinet Minister Lord Fowler, joined in, saying Cameron's vision of a Royal Charter to oversee a regulator was "always a bad idea".
The Labour and Lib Dem leaders are bracing themselves for a newspaper backlash after holding out for statutory underpinning of the new system.
Most newspapers - and most of the Conservative Party as well as some Labour MPs - are ardently opposed to any law to regulate the press.
Speaking after the talks broke down, Miliband accused Cameron of betraying a pledge he made to hacking victims that Leveson would be implemented unless it was "bonkers".
He said: "David Cameron's decision to walk away from the talks is an historic mistake.
"He has not just walked away from the talks, he has walked away from his own words."
Clegg said he would work "flat-out" over the next few days to build consensus with MPs from other parties on a "proper solution" to the issue.
"I was disappointed, and indeed surprised, that David Cameron has decided to walk away from cross-party talks, especially
when they were making some real progress as late as yesterday evening."
Cameron had been"wrong to abandon cross-party talks at this critical point", he added.
Lord Fowler's comments are a sign the Prime Minister could struggle to get his preferred method through the House of Lords even if they manage to clear the House of Commons.
They could well be defeated by MPs, with Labour and the Liberal Democrats both supporting the statutory underpinning of regulation proposed by Leveson.
The outcome is far from certain because many Tory MPs are in favour of the Leveson plan, while some Labour MPs are opposed to it.
Fowler, who is also a former Conservative Party chairman, said the charter was "always a bad idea because it puts power in the hands of the Government and it bypasses Parliament".
He said: "Somehow we have got to come to some agreement on this. But agreement is a long way away at the moment."
"I have yet to meet anyone who thinks the Royal Charter is the ideal way of dealing with this," he added.
Campaigners were furious after Thursday's talks broke down, with Hacked Off calling the failure to agree "a shameless betrayal of the victims of press abuse".
Jacqui Hames, a former police officer who was put under surveillance by the News of the World, said: "I am incredibly disappointed to see that David Cameron wants to walk away and go back to where we were - back to the press and media moguls bullying and harassing the public."
Actor and Hacked Off figurehead Hugh Grant claimed Cameron had been ordered by the editor of The Times to call off the talks - prompting critics to ask why the campaigner for press ethics was reporting a "rumour".
Bob Satchwell, executive director of the Society of Editors, said: "It is important that the genuine need for reform is not allowed to continue to be overtaken by high politics.
"There are issues with principles and practicalities of legislation and editors have a right to be concerned about where politics and a 'dab of statute' might take us years down the line as attempts to hijack important legislation on other matters have shown.
Cameron said the proposals for an independent body, established by Royal Charter, to oversee the system of press self-regulation would provide "the toughest regulation of the press that this country has ever seen".
Labour is expected to publish tomorrow its amendment for what was being described as a "Leveson-compliant" royal charter.