Police missed a number of chances to bring paedophile Ian Watkins to justice sooner, an investigation has found.
Between 2008 and 2012, South Wales Police did not adequately act on eight reports and three intelligence logs from six people about the former Lostprophets frontman's intentions.
Watkins was jailed for 35 years in 2013 after admitting 13 child sex offences.
The force accepted the Independent Police Complaints Commission's (IPCC) findings and "serious concern" raised.
IPCC Commissioner for Wales Jan Williams said: "This investigation raised the most disturbing concerns about the way in which reports of Ian Watkins' sickening child abuse were handled between 2008 and 2012.
"Watkins' arrest for his depraved activities followed only after an arrest for drugs offences, an added cause for serious concern."
The investigation was launched in 2013 following a referral by South Wales Police.
It found the force did not examine the mobile phone of Watkins' ex-girlfriend Joanne Mjadzelics in 2009, which had a message revealing his wish to have sex with children, and provided corroboration to other reports.
There was no evidence to show there was a lack of police action because of Watkins' celebrity status, however the belief that Ms Mjadzelics lacked credibility led to adequate steps not being taken to progress reports.
Several other individuals also came forward, some through Crimestoppers, with disturbing information about Watkins' behaviour, the IPCC found.
But it was the drugs arrest which led to further action.
"In my view, all the reports made were subject to a biased response, whether this was conscious or unconscious," Ms Williams added.
"Some were conflated with reports made by Ms Mjadzelics and were thus dismissed as lacking credibility.
"All those involved in responding accepted the initial sceptical view of Ms Mjadzelics' reports, demonstrating a lack of open-mindedness and professional curiosity.
"This continued until 'the right type of complainant came along'."
'Truly sorry'
South Wales Police Assistant Chief Constable Jeremy Vaughan said: "Today's report highlights a number of failings in which information about Watkins was investigated between 2008 and 2012 which the force entirely accepts and regrets.
"South Wales Police failed to listen and properly investigate information about Watkins offending behaviour, for this we are truly sorry."
Earlier this year, a detective sergeant was cleared of misconduct over the case and no further action was taken against two detective constables.
Ms Mjadzelics was cleared in 2015 of child sex abuse image offences. She claimed she had encouraged the singer to send the images in order to expose his criminality.
The IPCC has also previously released findings from investigations into how South Yorkshire Police and Bedfordshire Police handled allegations made to them about Watkins.
Wales' Children's Commissioner Sally Holland said: "It is disturbing to think that such a catalogue of basic errors can take place in Wales in the 21st Century, which led to a four-year delay in bringing Watkins to justice. There can be no defence or excuse."
