Suzanne Breen, northern editor of the Sunday Tribune, is refusing to reveal confidential information about her contacts with the Real IRA.
Ms Breen has been threatened with legal action if she fails to hand over notebooks, phones, computers and discs linked to two articles she has written about dissident republicans.
Detectives investigating the Real IRA murder of two soldiers at Massereene barracks in Antrim in March gave her seven days to comply with the request.
But Ms Breen says she won’t be handing over the information.
“Compromising sources undermines the freedom of the Press,” said the award-winning reporter.
“Journalists and police do different jobs.
“Our role is to put information into the public domain.
“If a journalist becomes a gatherer of evidence or a witness for the state, they cease to be a journalist.”
Ms Breen’s stance has drawn support from civil libertarians.
Professor Brice Dickson of Queen’s University’s School of Law said: “It is essential to the running of a healthy democracy that investigative journalists be allowed to go about their perfectly lawful activities without being impeded or constrained by police.”
A PSNI spokesman said: “Police will use every possible means to access information which may assist their inquiries.
“All victims of the Massereene attack, their families and the community expect and deserve nothing less.”