Sheriff Alastair Brown ruled evidence gathered by the vigilante group in the case of Paul Potelle was inadmissible and amounted to fraud.

He said the group had not acted in good faith and its actions had been “unlawful at all stages”.

Potelle, 47, had been charged with sending indecent social media messages to what he believed to be a 13-year-old girl and a child under 13, between May 22 and August 22, 2017.

The case was heard at Dundee Sheriff Court

The recipients of the messages were, in fact, adults who posed as children online.

They then travelled to Dundee to confront Potelle, who had to be taken into custody for his own protection, Dundee Sheriff Court heard on Thursday.

Sheriff Brown warned undercover work should be left to the police but rejected the argument that to use evidence from “paedophile catchers” would amount to a contravention of Article 8 of the European Convention on Human rights, which relates to privacy.

Sheriff Brown said: “I have reached the conclusion that the schemes operated were unlawful at all stages, and therefore the evidence is inadmissible.

“Put simply, what they did was fraud. They made a false pretence about themselves. They were knowingly dishonest in order to bring about a particular result.

“They then set out to induce him (Potelle) to continue to engage with messaging and to conduct himself in a way that would lead to a prison sentence.

“Their conduct in the exchange was calculating and manipulative. They then travelled to Dundee to confront Mr Potelle and he had to be taken to custody for his own protection.

“I have rejected the position that what was done, was done in good faith.”

Sheriff Brown said that he believed “personal gratification” was a “significant motivator” for the actions of one of the individuals posing as a “decoy”.

The sheriff added: “Internet crime is a serious issue, though it’s far more complex than they appear to recognise.

“Police Scotland do take it seriously but policing is a complex activity, which needs to be left to the police.

“They arrest them in a way that does not cause public disorder. They do not post photos of suspects on the internet.”

Sheriff Brown explained that Potelle was not guilty of indecent communications as he was communicating with adults who were consenting and even “hoping” for the communication.

The case has been continued until May 10 for the prosecution to decide whether they will appeal the ruling.

Potelle’s attendance has been excused.

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Scots judge brands ‘paedophile catchers unlawful at all stages’