The UK needs to pass a "shield law" to protect journalists and their sources from state grabs of their telecoms records and other intrusive surveillance, according to Gavin Millar QC.
He said that alarm bells should have been rung in 2008, when Thames Valley police bugged local newspaper journalist Sally Murrer in order to listen to her conversations with a police contact. The force failed to declare that she was a journalist when it sought outside authorisation.
And he said that abuses of police survillance powers have continued since then in the cases which have been highlighted by the Press Gazette Save Our Sources campaign - specifically the police phone records grabs against journalist at The Sun and Mail on Sunday who were not under suspicion of having broken the law.
"Ideally, I say, we need a shield law for journalists - a free-standing law that protects confidential information and confidential sources and that judges and law enforcement agencies cannot by-pass or miss because it is sitting there in front of their faces in black and white," Millar told the Society of Editors annual conference gala dinner.
Milton Keynes Citizen reporter Sally Murrer was charged with aiding and abetting misconduct in public office by receiving information from a police officer in 2008.
Thames Valley Police, Miller said, had gained authorisation under the Regulation of Investigatory Powers Act 2000 to bug the suspected officer's car and record their discussions, which is known in the Act as "intrusive surveillance".
But, said Millar, after Murrer was charged it emerged that the papers relating to the Chief Constable's authorisation and the approval of the authorisation by a Surveillance Commissioner, who was a retired judge, made no reference to the fact that she was a journalist or that the investigators wanted to identify a confidential journalistic source.
"This was a startling omission because the right of the journalist to protect the identity of such a source is strongly protected in our law and in European Human rights law," said Millar.
"It can only be overridden if a judge decides that there is an even more important public interest requiring the source to be identified - and that the evidence being sought cannot be obtained in some other way. Examples of such an overriding public interest might be the need fully to investigate terrorism or serious organised crime."
But the Surveillance Commissioner would not have known that these principles had to be applied in approving the authorisation - because the papers were silent about what was really going on, Millar said.
The prosecution was halted by an order at Kingston Crown Court recognising that this key evidence against Murrer and the officer was obtained in violation of fundamental journalistic rights, Millar said.
"The Murrer case didn't ring alarm bells. But it should have," he went on.
"First the charge - aiding and abetting misconduct in public office - was a novel one against a journalist. Previous unsuccessful attempts to prosecute journalists in this sort of situation had alleged that the journalist acted 'corruptly' in dealing with the source, within the meaning of the Prevention of Corruption Act 1906."
He said it was a vague charge, and added: "The underlying offence of the source, misconduct in public office, is a very old common law offence without the clear elements we see in modern statutory criminal legislation.
"Journalists doing their jobs exercise basic human and common law rights of freedom of expression. Indeed our law now recognises that it is the duty of the journalist to pass on information and ideas to the public on all matters of public interest.
"For this reason any criminal offences which they might be accused of committing in the process should be clearly defined and should contain explicit defences protecting the public interest aspects of their work.
"The charge faced by Sally Murrer in 2008 met neither of these important legal objectives."
In addition, it was clear that the Ripa powers were being misused.
"This was so even in terms of the Act itself - which only allows 'intrusive surveillance' where 'serious crime' is being investigated," Millar said, adding: "This was plainly not the case in R v Murrer - Sally's stories were good, solid local newspaper fare."
But it was also a misuse because police were not respecting the right to protect the source, he said.
"In the years since R v Murrer nothing has changed. In fact things have got worse - much worse," Millar said.
"Parliament should have removed the possibility of this obscure charge being laid against journalists in this situation. It should have debated whether there should be any criminal offence for journalists in dealing with sources and, if so, framed clear legislation so that everyone would know where they stand.
"This is the proper approach in a mature democracy where important rights are at stake.
"In the course of such a debate the difficult issues about criminalising payments to sources would have been considered by those we elect rather than being left to prosecutors and juries to grapple with.
"Instead this offence is now being charged regularly against journalists who, unsurprisingly, all seem to say the same sorts of things - 'we didn't know about this offence', 'we didn't know were breaking the law' and 'we were just doing our job'. The results of these prosecutions are, equally unsurprisingly, difficult to predict.
"Parliament should also have written the right to protect confidential journalistic sources into RIPA - along with explicit procedural safeguards for this important right.
"These should make clear that when investigating authorities want to use covert powers to identify sources they must go before a judge with a full account of the issues in the case. The legislation should explicitly require the application by the judge of the established legal principles on source protection.
"None of this has happened.
"Instead, I and fellow lawyers acting for the online news outlet the Bureau of Investigative Journalism have had to take a case to the European Court of Human Rights in Strasbourg - arguing that RIPA is incompatible with the right to protect sources. This should not have been necessary."
Millar said RIPA powers were still being misused to identify confidential sources, as had happened in relation to the Plebgate scandal and the Chris Huhne speeding points coverage.
"None of this would have happened if Parliament had done its job properly and protected the journalists' rights in the legislation," he said.
"Most worryingly of all we do not know how often and in what situations the police and other public authorities are using these covert powers against journalists.
"There is no information in the reports presented to Parliament about the operation of the legislation. And the police themselves cannot or will not say.
"Information about individual cases surfaces only sporadically, usually in a court case. I believe they have been used in many other cases, particularly in recent years.
"Again this is not what one would expect in a democratic society with strong traditions of press freedom."