Lord Justice Leveson called for the law which protects journalists from having to disclose material to the police, even if it has been obtained illegally, to be reconsidered.
His proposed changes could damage investigative journalism by putting off those who want to expose wrongdoing, such as the MPs' expenses scandal, from coming forward if they risk being named to the police.
Magnus Boyd, a partner at PSB Law, said: "I think the relationship between a journalist and his source here is the cornerstone of investigative journalism, the cornerstone of public interest journalism.
"I think we would need to think very carefully before you put anything in place that fettered that.
"I'm a claimant media lawyer and I'm always having a gripe about journalists not having to disclose their sources, but the principle here is greater than any one case.
"It needs to be protected, you have to protect that relationship."
Under the proposals, journalistic material would only be protected "if it is held or has continuously been held since it was first created subject to an enforceable or lawful undertaking, restriction or obligation".
Rupinder Bains, director of law firm Pinder Reaux, said: "It's unworkable."
Journalists who wanted to keep the name of a source who had provided information or a tip-off confidential "would have to have a contract", she said.
"It's very hard to have an oral undertaking and be able to prove that in court.
"If you wanted to keep their names confidential then you would have to have a contract."
A source who wishes to remain anonymous would have to put their name on a written contract which would also have to identify which story or tip-off the agreement was in relation to, she said.
Ms Bains went on: "If there are people who want to remain anonymous and they don't want to put their name to an undertaking, how is that going to work?
"The whole thing is very wishy washy, the whole 2,000 pages of it, it doesn't home in on anything.
"If people were waiting for something magical to come out it has not happened."
Lord Justice Leveson asked the Home Office to consider weakening the protections for journalistic material in the 1984 Police and Criminal Evidence Act (Pace), saying it was "remarkable" that Parliament might have provided greater protection for journalists' sources than for those covered by legal professional privilege.
But the judge admitted there may be problems with his suggestions having "far wider ramifications of which I have not been apprised".
"Before any conclusion can be reached on these issues, appropriate consultation will be essential," he said.
