A failure to act by police examining allegations of child abuse against Lostprophets singer Ian Watkins may have put a child at risk of further abuse for months, a report has found.
The IPCC said South Yorkshire Police did not take seriously claims Watkins had been sharing indecent images.
It said three officers would have faced misconduct hearings over their inaction but that all three had since retired.
Watkins was jailed for 35 years in 2013 after admitting 13 child sex offences.
South Yorkshire Police said it accepted the findings and admitted its handling of the matter was "far below the standard the public or we as a service would expect".
The Independent Police Complaints Commission (IPCC) said South Yorkshire Police were asked in March 2012 by colleagues in South Wales to meet Watkins' ex-girlfriend Joanne Mjadzelics after she made allegations that he had sent her an indecent image.
It said the request was initially wrongly allocated to its Safer Neighbourhoods team rather than the Public Protection Unit and, when Ms Mjadzelics produced her computer, the untrained officer neither viewed the image nor seized the device.
Two months later, on 3 May, Ms Mjadzelics took her computer to Doncaster police station again and again it was not seized.
Officers did not view the image until 11 May but concluded it was of an adult female and destroyed the laptop before Watkins was eventually arrested.
The IPCC launched an investigation into the force's handling of the allegations after Ms Mjadzelics complained officers had not examined her laptop properly, and that an officer had used inappropriate language towards her.
IPCC commissioner Jan Williams said: "Having taken into consideration the nature and seriousness of Ms Mjadzelics' allegations against Watkins, the inaction of some South Yorkshire police officers involved may have placed a child at risk of further abuse for several months.
"The evidence suggests there was a general view among officers at Doncaster that Ms Mjadzelics was not to be taken seriously, and consequently inquiries were not progressed as they should have been."
Assistant Chief Constable of South Yorkshire Police David Hartley said: "Our handling of this matter fell far below the standard the public or we as a service would expect, and I would like to offer my sincere apologies to Ms Mjadzelics as she received a service that was unacceptable with no support."
Watkins, from Pontypridd, pleaded guilty at Cardiff Crown Court to 13 child sex offences, including the attempted rape of a baby.
Ms Mjadzelics, from Doncaster, was cleared of child sex abuse image charges.