Lord Justice Leveson recommended on Thursday (November 29) "a genuinely independent and effective system of self-regulation" of the press, backed by a new statutory law.
However, Cameron has stated that he has "serious misgivings" about underpinning any press regulatory body with legislation.
Rowling - who spoke at the Leveson Inquiry last year - wrote in The Guardian: "I am alarmed and dismayed that the prime minister appears to be backing away from assurances he made at the outset of the Leveson Inquiry."
[Leveson with the report inside the QEII Conference Centre in Central London]
Arguing that Leveson's proposal for statute-backed self-regulation would "give everybody, whatever their degree of celebrity or their bank balance, a quick, cheap and effective way of holding the press to account", the Harry Potter author went on: "Without statutory underpinning, Leveson's recommendations will not work.
"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 Justice 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 relive extremely painful episodes on the stand in front of millions.
"Having taken David Cameron's assurances in good faith at the outset of the inquiry he set up, I am merely one among many who feel duped and angry in its wake."
Victims of press intrusion have launched a petition calling for Lord Justice Leveson's recommendations for media industry regulation to be implemented.