Thames Valley Police did not say Sally Murrer was a journalist when it applied to bug her conversations, her barrister has revealed.
This week, the force admitted it had used the Regulation of Investigatory Powers Act to bug the car of Murrer's alleged police source in December 2006.
This is the same act act which has been used by police to secretly obtain the phone records of a number of journalists.
Phone record grabs can be consented to internally by the police force concerned, but use of listening devices under RIPA must be externally approved.
According to Gavin Millar QC, who acted for Murrer, if Thames Valley Police had made clear in its RIPA application that the target was a journalist, the authorising authority would have had the chance to use the "correct, and very strict, legal test for overriding journalistic source protection".
For a bug application under RIPA, Millar said authorities need to get approval from high within the force - an assistant chief constable or chief constable - and this then needs to be approved by a Surveillance Commissioner.
Murrer was charged with conspiring to commit misconduct in a public office after a police officer allegedly leaked her confidential information, but evidence against her was ruled inadmissible after a five-day legal battle at Kingston Crown Court.
The Crown formally offered no evidence on seven out of eight counts on the indictment. The judge was asked to enter formal not-guilty verdicts on count eight.
A judge decided that her right to do her job as a journalist had been breached under the European Convention on Human Rights and evidence against Murrer, her police contact and a private detective could not be used in any trial.
"The fact that the judge had not been told the recording was to show a source-journalist relationship was the key reason that there was a breach of her Article 10 rights," Millar told Press Gazette this week.
"You will have seen in some of the coverage that a judge considering whether to override journalistic source protection has to rule that there is an overriding public interest in identifying the source.
"If the judge is not told that this is what the case is about (in the paperwork she or he receives) then she or he cannot apply the correct, and very strict, legal test for overriding journalistic source protection."
Millar, who has described described police use of RIPA to obtain journalists' sources as "completely illegal" under European law, said he believes that "this sort of thing happens in a lot of RIPA applications involving journalists or sources". He added: "I.e. the decision taker who grants the authority is simply not told by the investigating officers that they are trying to identify a journalistic source."
Thames Valley Police has rejected a Press Gazette Freedom of Information question about the number of times it has used RIPA against journalist on cost grounds, revealing that it has made in excess of 50,000 RIPA records from the last ten years.
A spokesperson said the force "does not record the occupation, as a matter of course, when acquiring data under RIPA".
The Home Office is set to consult on changes to the RIPA code of practice which could give journalists and their sources greater protection.