The Metropolitan police have applied for a court order against The Guardian and one of its reporters forcing it disclose confidential sources used in its acclaimed coverage of the phone-hacking scandal.
In a statement the force said its phone-hacking inquiry, Operation Weeting, was the one of its 'most high profile and sensitive investigations'and claimed the public interest was "protected by ensuring there is no further potential compromise".
"We pay tribute to the Guardian's unwavering determination to expose the hacking scandal and their challenge around the initial police response,'the Met said.
'We also recognise the important public interest of whistleblowing 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 MPS does not seek to use legislation to undermine Article 10 of anyone's human rights and is not seeking to prevent whistle blowing or investigative journalism that is in the public interest, including the Guardian's involvement in the exposure of phone hacking."
Responding to the news, The Guardian described the Met's behaviour as an 'unprecedented legal attack on journalists' sources", with editor Alan Rusbridger adding: 'We shall resist this extraordinary demand to the utmost".
NUJ general secretary Michelle Stanistreet said the court order was a 'very serious threat to journalists'and vowed to help fight the 'vicious attempt to use the Official Secrets Act to force journalists to disclose their sources on hacking".
'Attempts to confiscate information provided by sources violates the freedom of the press and damages the preconditions of an open and democratic society,'said Stanistreet.
"The protection of sources is an essential principle which has been repeatedly reaffirmed by the European Court of Human Rights as the cornerstone of press freedom and the NUJ shall defend it.
'In 2007 a judge made it clear that journalists and their sources are protected under article 10 of the Human Rights Act and it applies to leaked material. The use of the Official Secrets Act is a disgraceful attempt to get round this existing judgement."
The Guardian reported that the application was authorised by Detective-Superintendent Mark Mitchell from the professional standards unit, who alleged that Guardian reporter Amelia Hill had breached the 1989 Official Secrets Act.
The Guardian also reported that under clause five of the act, individuals can be prosecuted for passing on damaging information leaked to them by government officials in breach of section 4 of the same act, which includes police information 'likely to impede â€¦ the prosecution of suspected offenders'".
Executive director of the Society of Editors Bob Satchwell said: "This is outrageous, pointless and baffling. The Official Secrets Act is designed to protect national security so there is no justification in this case. The law, and particularly the Human Rights Act, is supposed to protect journalists."