Police and prosecutors were wrong to treat an investigative journalist who exposed a drug-taking doctor as a suspect and interview him under caution, the Crown prosecution Service has said.
The concession came from Crown Prosecution Service (CPS) chief executive Peter Lewis in a letter to the Mail on Sunday.
It follows concerns raised by the newspaper over an incident in which West Yorkshire Police officers investigating allegations of drug-taking by a senior doctor interviewed a journalist working for the newspaper under caution as a suspect and questioned him about whether he was committing voyeurism or had supplied drugs.
Lewis said in his letter that cases such as this should, as a matter of course, be referred to the CPS Special Crime Central Casework Division, in accordance with CPS guidelines on cases affecting the media.
A CPS spokeswoman said: "Had this case been referred in accordance with the guidelines, we are confident that the police would have been advised not to interview the journalist as a potential suspect."
Lewis told the newspaper in his letter: "The DPP and I are both very clear that investigative journalism in the public interest plays a vital role in a democratic society and this is clearly set out in the CPS guidelines to prosecutors on assessing the public interest in cases affecting the media."
The CPS spokeswoman said the case had now been reconsidered and West Yorkshire Police had been given advice about the CPS guidelines, and on the public interest protection afforded to journalists in law.
"Further advice has also been given in relation to the journalist. It is a matter for the police as to whether they wish to continue their investigation," she said.
The MoS used a secret camera to film an NHS surgeon taking Class A drugs before starting his shift.
The newspaper reported that detectives who questioned the reporter asked if he had supplied illegal drugs and if he had made the footage from the cameras which captured images of Dr Colin Ferrie snorting illegal substances for his own sexual gratification.
Section 67 (1) of the Sexual Offences Act 2003 makes voyeurism a criminal offence, which is committed by someone who, for his or her own sexual gratification, observes another person doing a private act without their knowledge or consent.
The Mail on Sunday said its lawyers had written to Director of Public Prosecutions Alison Saunders and West Yorkshire chief constable Mark Gilmore to express "grave concerns" about the case.
The Mail on Sunday uncovered the drug habit of Dr Ferrie, a consultant paediatric neurologist, and an expert in epilepsy, after a source approached one of its reporters last year.