The editor of the Spectator has pledged not to sign up to any new statutory watchdog of the press, ahead of the Leveson report tomorrow.
The moves comes as David Cameron and Nick Clegg read the advanced copy of the report before what is expected to be a dramatic and emotional session in the Commons.
In a lead article to be published in tomorrow's edition of the Spectator, Fraser Nelson wrote: "If the press agrees a new form of self-regulation, perhaps contractually binding this time, we will happily take part.
"But we would not sign up to anything enforced by government. If such a group is constituted we will not attend its meetings, pay its fines nor heed its menaces.
"We would still obey the (other) laws of the land. But to join any scheme which subordinates press to parliament would be a betrayal of what this paper has stood for since its inception in 1828."
The pledge constitutes a serious threat to Leveson's proposals if they back – as seems likely - statutory legislation to create an independent watchdog. If media organisations pre-emptively refuse to take part in any new regime, they may call Cameron's bluff, or prompt accusations of bullying and arrogance.
Meanwhile, Cameron and Clegg's respective teams are combing through the lengthy report behind closed doors. If they disagree on the findings, the Commons could be the scene of an unprecedented debate tomorrow in which the prime minister and deputy prime minister make contradictory statements.
The development comes as the public mood seemed to turn decisively against self-regulation of the press, with a new YouGov poll showing 79% of people favour "an independent press regulator established by law".
The poll cannot quite be taken at face value, however, after a Survation poll earlier this month found just 24% of the public want "new laws and regulations" as a result of the Leveson inquiry.
Eighty-six MPs from all political parties – but mostly Tories – wrote to the Telegraph demanding David Cameron hold off from statutory regulation of the press, regardless of Leveson's recommendations.
Among the signatories were Liam Fox, a figurehead for the Tory right on the backbenches, David Davis, former shadow home secretary, and Graham Brady, chairman of the backbench 1922 committee.
Labour's former home secretary David Blunkett, who writes a column for the Sun newspaper, is also a signatory.
Sources in Downing Street suggest Cameron is considering holding a free vote on the issue and allowing MPs to act according to their conscience.
That would be an attractive escape route for Cameron, who has been furiously lobbied on all sides by the press, MPs, pressure groups and phone-hacking victims as the clock ticks down to Thursday's publication of the report.
It would probably result in a go-ahead for the Leveson recommendations, with Liberal Democrats, Labour and at least 42 Tories signed up for tough actions on the press system.
Today's YouGov poll found just nine per cent of people believe in self-regulation of the press, while 86% believe there will be a further outbreak of law-breaking along the lines of phone-hacking if it continues.