The father of missing Madeleine McCann said legal backing for any new system was the "minimum acceptable compromise for me and for many other victims" and urged the Prime Minister to "do the right thing".
He told the BBC Radio 4 Today programme: "I would have liked to have seen a properly independent regulation of the press, whereas I think he has given the press another opportunity of self-regulation."
It should be made compulsory and measures put in place make journalists more accountable, he said.
He added: "But I do accept that full implementation of Lord (Justice) Leveson's report is the minimum acceptable compromise for me and I think for many other victims who have suffered at the hands of the press.
"The Prime Minister and our other elected politicians have an opportunity now to do the right thing. And if they do the right thing, for the public, then it will help restore a little confidence.
"I clearly respect his opinion but I personally disagree with the viewpoint and Lord (Justice) Leveson, as a senior law judge of our country, has made clear that what he is proposing is not a state-run press.
"It is a fine distinction but without the statutory underpinning this system will not work."
A man who won libel damages after his arrest over the murder of a woman has written to MPs asking them to back Leveson's recommendations.
About 2,000 editors will sign up to a new independent press watchdog, the chairman of the Press Complaints Commission has said.
The Prime Minister has taken a political gamble by holding differing views to the victims, the Leveson inquiry and a majority of MPs.