Police chiefs have been ordered by the Prime Minister's surveillance watchdog to reveal when snooping powers have been used to identify journalistic sources.
Sir Paul Kennedy, interim Interception of Communications Commissioner, has launched an inquiry into potential misuse of the Regulation of Investigatory Powers Act (Ripa) following reports forces have used the legislation to access reporters' telephone and email records.
His intervention comes after it emerged police investigating the Chris Huhne speeding points scandal secretly obtained the phone records of a journalist and one of his sources for the story, even though a judge had agreed that the source could remain confidential.
And it was previously revealed that the Metropolitan Police used the Act to obtain The Sun's newsdesk telephone records and those of its political editor, Tom Newton Dunn, to try to identify who had leaked the so-called Plebgate story involving former Tory chief whip Andrew Mitchell.
Announcing the inquiry, Sir Paul said: "I fully understand and share the concerns raised about the protection of journalistic sources so as to enable a free press."
The move also comes after the Press Gazette launched its "Save Our Sources" campaign, urging the commissioner to take action. A petition as part of the campaign, which was directed at Sir Paul, has attracted more than 1,000 signatures.
It recently emerged that a Kent police officer was granted authorisation to obtain the billing and call data of a Mail on Sunday journalist, alongside his source, who was later unmasked as a freelance journalist.
The pair, whose data were obtained from their landline and mobile phone service providers, had been in discussions with Constance Briscoe, the judge who was investigated by police over a false claim that she had not spoken to the press about the affair.
The Crown Prosecution Service (CPS) initially applied under the Police and Criminal Evidence Act (Pace), which enshrines the principle of journalistic privilege and allows journalists and their employers to make representations to the court to protect their sources.
But the judge ruled that while the Mail on Sunday did have to disclose material, it could do so with names of sources redacted.
Unknown to the Mail on Sunday, Kent police secretly went to the journalist's mobile phone provider and ordered the release of records using powers under Ripa, which does not require judicial approval.
Sir Paul said his office would report all findings to the Prime Minister, and publicly, to develop "clarity in relation to the scope and compliance of this activity".
In addition, it was reported the Home Affairs Select Committee has asked every police force in the UK to reveal how many times they have obtained the telephone and email records of journalists without their consent.
Keith Vaz, chairman of the committee, said he wanted a detailed breakdown of police use of Ripa powers to force telecoms companies to hand over phone records without customers' knowledge for all professions.
Speaking to the Press Association, Mr Vaz said: "It will alarm the public and it also alarms me that these powers are being used in this way."
Sir Paul is acting as interim commissioner while Sir Anthony May recovers from a serious accident that happened at the beginning of July.
Downing Street said David Cameron believed the "right oversight mechanisms" were in place for monitoring the use of Ripa, but stressed that his approach was to be "supportive of our tradition of investigative journalism".
The Prime Minister's official spokesman said Mr Cameron " thinks that strong and robust investigative journalism is absolutely essential. He is a strong supporter of the British media's long, proud tradition of that type of journalism".
Emma Carr, director of Big Brother Watch, said: "The Government needs to urgently address the fact that the Interception of Communications Commissioner has warned that spying powers are being over-used by some police forces.
"Quite simply, if the police can't get it right with the powers they already have then it is completely irresponsible for the Home Office to be planning on increasing those powers.
"The inadequacy and inconsistency of the records being kept by public authorities about how they are using these powers is woefully inadequate. Correcting this would not require new laws so it should not wait until after the election.
"If the Government fails to address these serious points, we can already know that there will be many more innocent members of the public who will be wrongly spied on and accused. This is simply unacceptable."
The Sun has called for a public hearing at the Investigatory Powers Tribunal into the way the Met accessed its journalists' records.
The newspaper's managing editor Stig Abell said the use of Ripa posed a "really serious risk to the freedom of the press".
He told BBC Radio 4's World at One: "We've asked for this Investigatory Powers Tribunal to look at it and to look at it in public.
"We know that it happened in connection with our political journalist Tom Newton Dunn and with our newsdesk.
"But one of the interesting issues that's emerging, not least with the Mail on Sunday yesterday, is the extent to which police are using these powers.
"Police are not having to go to court, (they) can ask a superintendent for the powers to access the phone records of journalists and thereby identify sources.
"That could be going on to a very wide extent and we are trying to increase the pressure so we can find out if that is indeed the case."
He added: "If you are going to seek to access information about journalists' sources you should be doing it in front of a judge, where the publisher or the broadcaster or the journalist themselves can make an argument and be aware of what is going on.
"What we have learned over the last few weeks is that it is potentially the case that police forces across the country have the ability, through merely the permission of a superintendent, to access the phone records of journalists and thereby identify sources."