Journalists have been urged to "wake up" to a new threat to press freedom.
A Home Office review could give the police new powers to seize journalistic material, NUJ general secretary Jeremy Dear has warned.
Counter-terrorism minister Tony McNulty is leading a full-scale review of the Police and Criminal Evidence Act (PACE) 1984, which currently gives police limited powers to seize journalistic material.
A consultation document suggests the part of the law which gives journalistic material special protection, sections 9-14, may need to be updated "to meet 21st [century] challenges in tackling crime ".
Currently police must apply to a senior judge to obtain journalistic material, prove it relates to a serious arrestable offence and that they have tried all other methods.
Journalists have the right to argue against the order.
The Northern Ireland version of PACE was amended last week, giving police there increased powers to take and withhold documents.
The Northern Ireland Policing (Miscellaneous Provisions) Order 2007 gives any police officer carrying out a lawful search the power to take any physical or electronic document relating to the "commission or proposed commission of serious crime" for a period of up to 48 hours.
An officer of the rank of chief inspector or above can authorise documents to be held for 96 hours.
Dear said: "At a time when freedom of information legislation is already under attack and there are widespread concerns about civil liberties it is vital the media industry responds robustly to such a worrying consultation.
"In the current climate, hard-won and vital rights to protect sources, to confidentiality, to protect material and equipment from seizure potentially are under threat — and unless we wake up to these potential threats to journalism, freedom of the media to act in the public interest could be seriously curtailed.
"There must be no diminution of the rights of journalists and no weakening of the requirement on the police to go before a judge to make their case or the right of the journalists to mount a public interest defence to such a case."
News International's senior legal manager Tom Crone said: "It's slightly typical of this particular government… piece by piece they are making dramatic inroads into what have always been traditional liberties.
It's something we will be looking very carefully at with a view to opposing anything that is wrong."
Society of Editors director Bob Satchwell said: "Of course terrorism requires special action but theGovernment and other authorities should also keep in mind the fact that professional reporters frequently help the cause. Any interference would reduce the effectiveness of journalism in exposing terrorism."
PACE was amended in 2005 as part of the Serious and Organised Crime Act to allow multiple warrants to be issued and to reduce "operational delay"
during police investigations "when entry to premises was required to protect and secure evidence".
The new consultation paper promises to "further rationalise" warrant application processes.
• The consultation document can be viewed at www.tinyurl.com/2xrbhr and submissions should be sent to pacereview@ homeoffice.gsi.gov.uk. The consultation period ends on 31 May.