Britain's security services are set to clash with parliament and the country's powerful press over revelations that police secretly used a controversial law to identify journalists' sources, triggering a wider debate about media freedom in the digital age.
Detectives used a law usually reserved for terror suspects to find out the sources for scoops in The Sun and the Mail on Sunday that led to the downfall of two senior officials.
Other cases have since emerged, although the full scale of the practice is unknown since police do not need to go through a court under the Regulation of Invest Powers Act (RIPA).
In the case of The Sun, the phone records of political editor Tom Newton Dunn were accessed to find out which officers were behind allegations that Andrew Mitchell, the government's senior representative in parliament, swore at police.
The tabloid had used the material in a front-page story.
"It was a fairly simple process for them to identify which telephone numbers could be linked to police officers," said Dominic Ponsford from the Press Gazette trade magazine, which is running a "Save Our Sources" campaign.
- 'Nobody above law': police -
RIPA was introduced in 2000 to take account of advances in technology and to comply with European law and specifies when authorities can access communications.
But a legal challenge has now been launched in the European Court of Human Rights on behalf of the Bureau of Investigative Journalism, which claims that the law is being used in a way that contravenes the right to free speech.
"There has been a huge amount of case law at the European Court to say that as part of that right to freedom of expression, the confidentiality of journalist sources has the highest possible protection in law," Ponsford said.
Journalists appear to have the backing of Home Secretary Theresa May, the interior minister, who is pushing a law restricting its use to cases of serious crime, and Paul Kennedy, the government's interception of communications commissioner, who has launched a review of the law.
But Metropolitan Police assistant commissioner Mark Rowley insisted the force would continue to use the law.
"Nobody should be above the law, whether it's a member of the public, whether it's a police officer, whether it's a journalist. We should be able to investigate and pursue any one of those," he told the BBC.
- 'Asymmetric battle' -
The country's newspapers boast of a tradition of holding institutions to account, but now complain that their huge resources and technical know-how are making it an uneven battleground.
"Journalists must remain arrows and not the target," Seamus Dooley, secretary of Ireland's National Union of Journalists, recently told a conference on journalism in the age of mass surveillance, hosted at The Guardian newspaper's London headquarters.
"The message must be: we are watching Big Brother," he added.
Guardian reporter Luke Harding warned the conference that it would remain "an asymmetric battle unless you have the tech skills of Edward Snowden" -- the former intelligence operative who blew the whistle on mass US surveillance.
Whistleblowers remain the Achilles heel of the authorities, and newspapers fear that they will be put off coming forward in the future if their anonymity can not be guaranteed.
"Our work as journalists would be over," Guardian editor Alan Rusbridger told the London conference. "The protection of sources is sacrosanct."
- Calls for judicial oversight -
As the law stands, a senior police officer can sign off a request to access someone's phone records, although not the actual content of the call or message.
"We're just saying the police shouldn't be allowed to mark their own homework, they need external oversight," said Ponsford, arguing also that regulators had failed to grasp the amount of information that can be gleaned from metadata.
In a message delivered to the London conference, Snowden said the answer lay not in the law but in more stringent technological safeguards.
Others, including Harding, argue that only a return to the pre-digital world of notepads and clandestine meetings in public spaces can deflect the glare of Big Brother.