More than 50 phone hacking victims have urged David Cameron not to reject calls for the media to be independently regulated if the Leveson Inquiry into media ethics suggests it.
They have called for an assurance that the Prime Minister would consider Lord Justice Leveson's recommendations with an open mind and that he had not already decided upon a system of continued self-regulation by the press.
The Hacked Off campaign - which represents the victims of media intrusion - expressed concerns after newspaper reports suggested Mr Cameron was preparing to reject statutory intervention in the regulation of the press, even if it was strongly recommended by Lord Justice Leveson.
The group sent an open letter to PM signed by victims of the 7/7 attacks, members of the Hillsborough Justice Campaign, and celebrities including Hugh Grant, Jude Law and Charlotte Church.
It reads: "It is highly regrettable to us that these articles, and supporting comments from senior Conservative Party figures, have sought to undermine the work of the inquiry and to threaten any recommendations it may make for effective regulation of the industry.
"To remind you once again - you said that the test of the future system of press regulation is not whether it suits the politicians or their friends in the press, but rather the public interest - including the need of members of society to be free from illegal and unethical press practices. Do we have those reassurances?"
Actor Hugh Grant told Sky News why it is important for changes to the regulatory system to protect ordinary citizens.
He said: "What matters is that people like the families of the Hillsborough victims.
"The Watsons, whose son committed suicide because his murdered sister was trashed in the tabloid press, it is people like them who matter.
"You could do whatever you like to me provided those people are protected and the democracy of this country is protected.
"No one wants a state-run media but what we've had to put up with for 40 years is a media-run state."
Hacked Off director Brian Cathcart added: "The victims of press abuse who signed this letter are alarmed that, before Lord Justice Leveson has even had the chance to report, it is reported that his proposals will be rejected.
"It is hard to believe that the Prime Minister, who after all set up the inquiry, could really have taken such a decision.
"The judge has spent a year investigating press culture, ethics and practices. His recommendations, when published, surely deserve to be considered with open minds and with the greatest seriousness?"
Lord Justice Leveson's inquiry was set up by Mr Cameron last year in the wake of the phone-hacking scandal that led to the closure of the News Of The World.
He is expected to announce the findings of his inquiry in the next few weeks.