The inaction of South Yorkshire police officers left a child at risk of being abused for months by the former Lostprophets frontman Ian Watkins, the police watchdog has said.

The Independent Police Complaints Commission said the force had failed to take seriously complaints made about Watkins by his ex-partner between March and May 2012.

It said three South Yorkshire police officers would have faced gross misconduct cases but had since retired after 30 years’ service.

Watkins was jailed for 35 years in December 2013 after pleading guilty to a string of sex offences against children, including the attempted rape of a baby. The IPCC said on Friday that South Yorkshire police must improve how it investigates child sexual abuse after finding it left children exposed to abuse by Watkins before he was jailed.

The criticism of the force comes five years after the Rotherham child sexual abuse scandal and after years of repeated warnings that South Yorkshire police needed to improve its handling of the cases.

IPCC criticism followed an investigation into complaints by Joanne Mjadzelics, the former lover of Watkins, who reported him to police and social services in 2008, 2009, 2011 and 2012 about his child sex crimes. Mjadzelics, who was herself cleared of child sex abuse image offences in January 2015, alerted the police five times to allegations against Watkins between 1 March 2012 and the following three months.

On three occasions, she took a laptop to Doncaster police station and said it contained an indecent image of a child sent to her by Watkins. However, the image was not viewed by specialist child protection investigators at any point, the IPCC found.

On the third visit to Doncaster police station, officers viewed the image but told Mjadzelics that the alleged victim was an adult female. The laptop was later destroyed prior to Watkins’ arrest.

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.” Williams said there was a “general view” among officers in Doncaster that Mjadzelics was “not to be taken seriously” and therefore enquiries were not progressed as they should have been.

The investigation found that a request in early March 2012 from South Wales police for assistance with Mjadzelics’ allegations was initially allocated to South Yorkshire police’s safer neighbourhood team, rather than specialist child sex abuse investigators. This meant that a police officer with no training in child sex abuse, or in the handling of evidence in computer-related offences, was assigned an initial meeting with Mjadzelics at Doncaster police station.

The officer did not view any image and there was “no subsequent determined effort to ascertain whether Ms Mjadzelics had evidence of child sexual abuse in her possession”, the IPCC said.

Williams added: “It is concerning that a neighbourhood police constable without specific training or support, rather than an officer from a specialist team, was expected to view and make judgment on a potential image of child sexual abuse. South Yorkshire police did not handle a request for assistance from South Wales police thoroughly. I have recommended they create a policy document setting out what is expected of officers in collaborating on serious offence investigations.”

The IPCC found that, in May 2012, specialist South Yorkshire officers were instructed to seize the laptop – said to contain images of child sexual abuse – at a further meeting with Mjadzelics and to take a statement, but neither action was undertaken.

The watchdog added: “On viewing the relevant alleged close-up indecent image of a child at the meeting, officers believed it to be of an adult female.”

The IPCC said there was also sufficient evidence that a police constable involved could have made inappropriate remarks to Mjadzelics about her motive for making allegations against Watkins. This officer, who has not been named, will be summoned to a misconduct meeting over the alleged inappropriate remarks, the watchdog said.