Police should be barred from accessing journalists' phone records without the approval of a judge, Nick Clegg said.
The Deputy Prime Minister said he was pressing Home Secretary Theresa May to strengthen the protections offered to the media after forces admitted using powers to trace the sources of stories.
Media bodies have voiced concern about a draft code of practice which would allow police forces to continue to access journalists' phone records without external authorisation.
Published this week by the Home Office, it states that communications data - such as the timing of and participants in phone or email exchanges, but not their content - is not subject to "professional privilege".
But it says that officers seeking data relating to "sensitive" occupations like journalists, doctors, lawyers, priests and MPs should note the fact when applying for permission.
The National Union of Journalists is calling for a new "shield law" to protect reporters' communications from state snooping.
Announcing a review of the application of the Regulation of Investigatory Powers Act (Ripa) in October, Mrs May said the code would be revised "to make clear that specific consideration must be given to communications data requests involving those in sensitive professions, such as journalists".
Mr Clegg said Tory coalition colleagues were holding off until the publication of a report next month by the Interception of Communications Commissioner but made clear he felt the move was needed.
"Unless that commissioner's report comes up with something dramatically different to what I expect I will stick to the view ... then we really should move now towards a position where the police have to seek a sign-off from a judge before they go after journalists in the way they have," he said on his weekly LBC radio phone-in.
"Ripa is not there for the police to go after the communications, information, data that journalists might have got totally properly as part of their profession.
"Whilst I don't always get the best press from my friends in the media I will defend journalists' right to my last breath as an old-fashioned liberal to go about making life uncomfortable for people. That is part of what journalism is about.
"It's keeping people on their toes. It is finding out stuff that other people want to hide. It's seeking out the truth that is otherwise being hidden.
"And you can't do that if journalists are constantly worried that the police are going to arbitrarily bear down on them.
"That's why I think there should be a judge sign-off."
Hundreds of journalists have signed a Save our Sources petition begun by trade magazine the Press Gazette, after police forces admitted accessing the phone records of reporters on papers including The Sun and Mail on Sunday in order to track down the sources of stories.
There is concern that police and other authorities may have been turning to Ripa, which permits access to communications data with the approval of a senior official within the organisation, because it is simpler than using the Police and Criminal Evidence Act (Pace), which requires a court order.