By Dominic Casciani
Protesters: Arms Trade campaigners regularly pickett fairs
The Court of Appeal has limited police powers to keep pictures of protesters in case they go on to break the law.
Judges said police had been wrong to retain pictures of a lawful arms trade activist who was not suspected of any criminal offence.
The Metropolitan Police said they acted reasonably in retaining pictures of the campaigner, Andrew Wood from Oxford.
Welcoming the judgement, Mr Wood said that the onus was now on the Met to review its retention policies.
The court has told the Met to destroy the photographs if it does not challenge the ruling in the House of Lords. The force has indicated it will not appeal.
Mr Wood argued that police had harassed him and infringed his right to privacy when he lawfully attended a company's annual general meeting (AGM) meeting in April 2005.
Reed Elsevier, a global publisher, had become the parent company of an arms fair exhibitions organiser called Spearhead.
Mr Wood and other campaigners bought shares in Reed Elsevier so they could attend the AGM and ask directors to justify this move.
Following the meeting, Mr Wood said Metropolitan Police officers openly photographed, questioned and followed him. Police did not accuse him of breaking the law and he was not arrested.
Mr Wood said the incident had been alarming and the police quickly established he was a law-abiding member of society.
Last year the High Court agreed with the Metropolitan Police that its actions were reasonable because officers need to detect crimes that may have occurred in the past or may do so in the future.
But overturning that ruling, Lord Justice Dyson said: "The retention by the police of photographs taken of persons who have not committed an offence, and who are not even suspected of having committed an offence, is always a serious matter.
"The only justification advanced by the police for retaining the photographs for more than a few days after the meeting [attended by Mr Wood] was the possibility that the appellant might attend and commit an offence [at a later arms fair].
"Even if due allowance is made for the margin of operational discretion, that justification does not bear scrutiny ..."
Lord Collins of Mapesbury added that the substantial police presence that confronted the arms campaigners had a "chilling effect" on those seeking to protest lawfully. A third dissenting judge said the Met had acted reasonably.
The judgement does not ban specialist police cameramen and photographers, known as Forward Intelligence Teams, but it does say the long-term retention of their pictures must be justified on a case-by-case basis.
Crucially, the ruling echoes a landmark privacy decision against the England and Wales DNA database at the European Court of Human Rights.
That judgement last December said police could not justify keeping forever the DNA profiles of people who were never charged with a crime.
Mr Wood, who was represented by campaign group Liberty, welcomed the judgement saying the police would now have to review its retention policies.
"The police don't just uphold the law - they must abide by it. They now have a duty to assess the photographs they keep," he told the BBC.
"If there is no criminal investigation then this ruling surely means they must get rid of the image. I expect them to go through photos and test them against the law. "