Best-selling novelist JK Rowling said she was "alarmed and dismayed" at David Cameron's response to the Leveson report.
The Harry Potter author, who gave evidence about press intrusion into her family's privacy to the inquiry, argued that without statutory underpinning Lord Justice Leveson's recommendations would fail.
And she described how she had been left feeling "duped and angry" by Mr Cameron's hesitance to "cross the Rubicon" and introduce a press law.
JK Rowling has expressed disappointment in David Cameron's response to the Leveson Report
She said: "I thought long and hard about the possible consequences to my family of giving evidence and finally decided to do so because I have made every possible attempt to protect my children's privacy under the present system, and failed.
"If I, who can afford the very best lawyers, cannot guarantee the privacy of those dearest to me, what hope did the Dowlers, the McCanns and the Watsons ever have of protecting their own children and their own good names?"
"Without statutory underpinning Leveson's recommendations will not work," she added.
"We will be left with yet another voluntary system from which the press can walk away.
"If the Prime Minister did not wish to change the regulatory system even to the moderate, balanced and proportionate extent proposed by Lord Leveson, I am at a loss to understand why so much public money has been spent and why so many people have been asked to re-live extremely painful episodes on the stand in front of millions."
Giving evidence in November last year, Rowling told the inquiry that journalists had driven her out of her home and targeted her five-year-old daughter at school.
The 47-year-old - whose characters in the Harry Potter series include an unscrupulous tabloid journalist called Rita Skeeter - was particularly critical of newspapers that published details about her homes that would allow people to find where she lived.