The Organisation for Security and Co-operation in Europe, which usually polices elections to ensure against human rights abuses, said that the phone hacking scandal should not be used as an "excuse" to restrict free speech.
The intervention was made after the Prime Minister was accused of putting together a “grubby deal” after he dropped his opposition to use legislation to underpin a new system of press regulation.
The Labour and Liberal Democrat leaders met with a Conservative minister in secret talks which lasted until early Monday morning to hammer out a deal which could see the press regulated by a new Royal Charter.
Those who refuse to sign up, including websites, now face “exemplary damages” if they are taken to court.
Several of the country’s biggest newspaper groups, including Telegraph Media Group, issued a statement expressing their initial concerns over the proposals.
There is growing speculation that some newspapers and magazines may boycott the new system amid claims it violates fundamental human rights.
Mr Cameron unveiled the plans in Parliament as several MPs - exposed for their own failings in the media - welcomed the crackdown.
In a statement, a spokesman for the Organisation for Security and Co-operation in Europe said: “A government-established regulatory body, regardless of how independent it is intended to be, could pose a threat to media freedom.