A key adviser to Lord Justice Leveson claims his last resort option for compulsory press reform laws would be illegal because it would "coerce" newspapers into holding higher standards than anyone else.
Shami Chakrabarti, one of six assessors who worked with the judge on the inquiry, told the Mail on Sunday she could not support any legislation that was forced on the press and claimed it could breach the Human Rights Act.
Lord Justice Leveson called for the industry to devise an independent regulatory body but wants it then to be backed up in law.
But the director of civil rights group Liberty warned the report's nuclear option of compulsory regulation if the press failed to sign up voluntarily could have "serious unintended consequences".
"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.
"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."
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.
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."
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.
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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.
