David Cameron warned that he has "serious concerns and misgivings" about the prospect of legislation on press regulation.
Responding to Lord Justice Leveson's report on media ethics, the Prime Minister broadly welcomed the principles he 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.
"I have some serious concerns and misgivings about this recommendation," he 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," he said.
"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.
"The danger is that this would create a vehicle for politicians, whether today or some time in the future, to impose regulation and obligations on the press, something Lord Justice Leveson himself wishes to avoid," he said.
"I believe there may be alternative options for putting in place incentives providing reassurance to the public and ensuring the Leveson principles of regulation are put in place, and these options should be explored."
He stressed however that the current regulatory system was "not an option" and said that he was inviting Deputy Prime Minister Nick Clegg and Labour leader Ed Miliband to join him in cross-party talks on how best to proceed.
"Let me be clear, a regulatory system which complies with the Leveson principles should be put in place rapidly. I favour giving the press a limited period of time in which to do that," he said. "While no one wants to see full statutory legislation, let me state the status quo is not an option."