London (CNN) -- Police in London have applied for a court order under the Official Secrets Act to try to force the Guardian newspaper to reveal confidential sources who have provided information on the phone-hacking scandal.
The Metropolitan Police Service (MPS) said it was asking for the order against the paper and one of its reporters to seek evidence related to a possible breach of the Official Secrets Act and legislation governing conduct in public office.
The legal step is about "seeking to identify evidence of potential offences resulting from unauthorised leaking of information," in connection with the police investigation into alleged phone hacking, a Metropolitan Police statement Friday said.
In a front-page story Saturday, the Guardian described the move as "an unprecedented legal attack on journalists' sources."
The story quotes the paper's editor, Alan Rusbridger, as saying: "We shall resist this extraordinary demand to the utmost."
The National Union of Journalists (NUJ) also condemned the police action in a statement on its website Saturday, describing it as an attempt to violate press freedom.
NUJ Secretary General Michelle Stanistreet said: "This is a very serious threat to journalists and the NUJ will fight off this vicious attempt to use the Official Secrets Act to force journalists to disclose their sources on hacking.
"Journalists have investigated the hacking story and told the truth to the public, they should be congratulated rather than being hounded and criminalised by the state."
The Guardian has taken a lead in reporting on the scandal around alleged phone hacking at the News of the World newspaper, which may have affected scores of celebrities, politicians and victims of crime.
News of the World was shuttered in July amid public outrage that followed the Guardian's July 4 accusation that the voicemail of missing teenager Milly Dowler, later found murdered, was hacked by the paper's reporters.
More than a dozen people have been arrested and bailed so far in the course of the Metropolitan Police investigation into the allegations, codenamed Operation Weeting. A second inquiry is looking at claims that police were bribed.
The Metropolitan Police statement said it had a responsibility to ensure there were no further leaks from Operation Weeting, as a sensitive and high-profile investigation.
"The MPS can't respond to the significant public and political concern regarding leaks from the police to any part of the media if we aren't more robust in our investigations and make all attempts to obtain best evidence of the leaks," it said.
"We pay tribute to the Guardian's unwavering determination to expose the hacking scandal and their challenge around the initial police response.
"We also recognise the important public interest of whistle blowing and investigative reporting, however neither is apparent in this case. This is an investigation into the alleged gratuitous release of information that is not in the public interest."
The phone-hacking scandal has forced two top police officers to resign and put Prime Minister David Cameron under pressure for hiring another former News of the World editor, Andy Coulson, to be his spokesman. Coulson resigned from that job earlier this year.
CNN's Bharati Naik and Laura Smith-Spark contributed to this report.