Mainstream journalistic standards could plunge if laws are not enforced against bloggers, Tweeters and websites, Lord Justice Leveson warned on Thursday.
In an address as part of his Australian lecture tour, the media ethics inquiry judge said journalists might be tempted to cut corners or break the law to "steal a march" on their online competitors.
Lord Justice Leveson told the University of Melbourne there was a "pernicious and false" belief that the law did not apply to the internet.
Newspapers that move wholly online could relocate their bases overseas to dodge UK laws, although that was unlikely in the near future, he said.
"In order to steal a march on bloggers and tweeters, they might be tempted to cut corners, to break or at least bend the law to obtain information for stories or to infringe privacy improperly to the same end," he said.
"It may encourage unethical and, potentially, unlawful practices to get a story. The effect then is a indirect one, and one which lies behind the headline and the front page scoop.
"In a culture which sees some act with impunity in the face of the civil law, and the criminal law, a general decline in standards may arise."
He called for creative thinking and international co-operation to tackle the problem.
"It might be said that if we facilitate or condone breaches of the law, and thereby weaken the rule of law by failing to act and to recognise judgments and court orders which emanate from other countries, we encourage the weakening of the rule of law at home too," he said.
"If we are to ensure that appropriate standards are maintained, we must meet those challenges, and ensure that the media not only remains subject to the law but that it is not placed at a disadvantage where the enforcement of the law is concerned.
"We will therefore have to think creatively about how we ensure that the law is capable of equal application, and is applied equally and fairly, against the mainstream media and bloggers, tweeters and other amateur online journalists."