The police prosecution of the former girlfriend of ex-rock star and convicted paedophile Ian Watkins, which last week ended in her acquittal, was an attempt to “cover up” their failure to investigate the singer, according to her lawyer.

Michael Wolkind QC has launched a devastating attack on the failure of three forces to investigate the repeated warnings of his client, Joanna Mjadzelics, that Watkins was a paedophile. He accused the forces of being prejudiced against her because she was a former sex worker and revealed they had shared intelligence wrongly suggesting that she had been sectioned under the Mental Health Act.

“As a sex worker, it made it easy for the police to discount her,” said Wolkind. “It was a very lazy stereotype. They didn’t think, ‘oh, she’s also been special constable or has been in the military police or worked in a bank’.” He said at first the police had thanked Mjadzelics for helping them to jail Watkins. They later charged her with possession of images of child abuse.

“They phoned her up and said: ‘we can’t thank you enough: without you, we would not have been able to prosecute him.’ And then they put her on trial. It was a mysterious prosecution, really hard to justify.”

Her information led to Watkins being jailed for 35 years. Two women accomplices were given sentences of 17 and 15 years respectively. “That’s 66 years of public service that she gave,” said Wolkind, “for which they charged her. It was a dirty prosecution. She was on a mission to save children and babies from abuse and the police were on a mission to cover up their neglect.”

Files passed between the three police forces said Mjadzelics had been sectioned but this was untrue. The Independent Police Complaints Commission is investigating the failure to look into her concerns.

Wolkind said Mjadzelics had drawn on her background to amass evidence on Watkins after police refused to investigate. “She was in love with him, which made it even harder. She tolerated his fantasies and, as an ex-sex worker, she was used to fantasies. These equipped her later to follow a script and give him what he wanted but she tripped into the sickest of worlds.”

He attacked the way in which the case against his client was presented in court, where she was presented as a vengeful woman who was prepared to expose Watkins after their relationship began to flounder.

A transcript shown in court appeared to show that Mjadzelics had initiated a conversation about raping children.

“But when you went back to the beginning of the conversation, it turns out, of course, that he introduced the subject of little boys,” Wolkind said. “She enthusiastically takes it up, not on a personal level but in the hope of him giving himself away. The police selected which bits they showed.”

Mjadzelics’s efforts to procure evidence against her lover were hampered by the fact that he destroyed much of the material. Watkins kept a tape marked “Jo paedo filth” in which the pair discussed his fantasies, potentially as a way to incriminate her, Wolkind believes.

The QC said he had never doubted that his client would be found innocent because it would become clear to the jury that she had possessed the images of child abuse “for legitimate reasons” -to help build a case against Watkins. However, he revealed that his client had been the subject of an online hate campaign. “There was a lot of malice and ignorance out there. She is fragile; she’s been fragile for a long time.”