A 13-year-old boy was allowed to visit the home of a known paedophile for two hours because senior police officers did not want to jeopardise an undercover operation, it has been claimed.
Surveillance officers asked for permission to intervene but their superiors in Greater Manchester Police (GMP) ordered them to maintain observation.
The operation, codenamed Nixon, did not result in any convictions and a misconduct investigation was only launched three years later in 2014 after a whistleblower contacted the Independent Police Complaints Commission, since renamed the Independent Office for Police Conduct.
However, both the IPCC and the GMP decided not to take formal action against the officer in overall charge of the operation, Dominic Scally, and a second senior detective.
Detective chief superintendent Scally has since been promoted to head of counterterrorism policing for the north west of England.
Further details of the incident emerged after Noonan was jailed last month for 11 years on 13 charges of historical sex offences against four young boys.
The IPCC investigation focused on whether officers "took appropriate action to mitigate any risks to the public" but said there was no suggestion that anyone was harmed while the boy was in Noonan's flat.
It said in its findings: "Officers witnessed the suspect enter a premises with two young people and remain there for a substantial period of time. A decision was made for the officers to maintain their observation and enquiries.
"The two officers leading the investigation were quickly made aware of the situation and made a conscious decision not to intervene and to maintain the observation of the subject."
An investigator concluded that the allegation of a "cover up" by the GMP was unfounded and decided there was no case to answer for gross misconduct.
There were however criticisms of deficiencies in the planning of the operation and a failure to formally record the potential risks.
The IPCC referred the case back to the police force for a formal misconduct hearing, which took place in March 2017.
Greater Manchester Police said in a statement that a misconduct hearing found "both officers' actions should be dealt with as a performance matter with appropriate action plans put in place."
The Times reported that senior officers were given specialist advice to send local uniformed police to the flat to investigate on the basis they had received a call from a concerned member of the public.
Surveillance officers also asked to carry out a safeguarding intervention, but were told to stay at their posts. The GMP said it would not comment on the claim senior officers decided not to intervene because of concerns about jeopardising the covert operation.
The Noonan family are notorious in Manchester and featured in the 2008 film A Very British Gangster.
Following Dominic Noonan's conviction for sexual assault, the police said he "used his notoriety to groom young vulnerable boys until his behaviour became normalised." He was already serving a prison sentence for arson and conspiracy to blackmail.