A key adviser to Lord Justice Leveson claims his last resort option for compulsory press reform would be illegal as it would "coerce" newspapers into holding higher standards than anyone else.
Shami Chakrabarti, director of civil rights group Liberty, told the Mail on Sunday any legislation forced on the press could breach the Human Rights Act.
Lord Justice Leveson called for the industry to devise an independent regulatory body but wants it to be backed up in law.
"We were chosen as advisers because of our areas of expertise," she told the Mail on Sunday.
"Mine is human-rights law and civil liberties. In a democracy, regulation of the press and imposing standards on it must be voluntary.
She suggests the report threatens Article 10 of the European Convention on Human Rights, which states: "Everyone has the right to freedom of expression. This right shall include freedom to hold opinions and to receive and impart information and ideas without interference by public authority and regardless of frontiers."
She told the Mail on Sunday: "A compulsory statute to regulate media ethics in the way the report suggests would violate the Act, and I cannot support it. It would mean the press was being coerced in being held to higher standards than anyone else, and this would be unlawful."
Of Hacked Off, she told the Mail on Sunday: "I understand that people who have been wronged want action. But they should be interested in outcomes, rather than particular processes. The outcome they should be seeking is a free and vibrant press with access to justice for the public when things go wrong."
Labour leader Ed Miliband has said David Cameron must back Lord Justice Leveson's proposal for a new law to regulate the press by Christmas.
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."
Ms Chakrabarti told BBC 1's Andrew Marr Show: "I support the Leveson plan for independent self-regulation of the press."
She said she supported the "carrots and sticks" in the report that meant press who signed up to a new watchdog would be subject to lower penalties than those who didn't.
"The bombshell, or the difference, is what do you do if people don't join the club or don't set up a club and Leveson doesn't want compulsory regulation of the press but he says if they don't play ball politicians may have to consider it. That is where I get off the bus."
Ms Chakrabarti said Labour leader Ed Miliband "was hasty" in declaring his full support for the recommendations made in the 2,000 page report unveiled earlier this week and criticised Miliband saying he could not have possibly had time to read the report.
David Cameron has indicated he plans to spike the so-called "Leveson law", which would back up the new independent regulator with statutory underpinning, warning he has "serious concerns and misgivings" about legislative action.
But he faces intense pressure to legislate from victims of press intrusion, the public and other party leaders.
The Prime Minister, who called for the press to be given a some time to now get its house in order, will warn Fleet Street it must take swift action to set up an independent press watchdog in crunch talks with the industry next week.
He will "drop in" on a meeting with editors being hosted by Culture Secretary Maria Miller next Tuesday and call for a timetable to be set for creating the new organisation, according to Downing Street.
The report has ratcheted up Coalition tensions with the Liberal Democrats saying they are prepared to back the regulatory system and slapping down Tory claims that draft laws were only being drawn up to demonstrate the proposals were unworkable
Lord Justice Leveson condemned the "culture of reckless and outrageous journalism" that dominated sections of the press for decades as he unveiled the findings of his 16-month inquiry earlier this week.
The Appeal Court judge called for a new watchdog with statutory underpinning to be given the power to require prominent apologies and impose fines of as much as £1 million.