“You don’t promote someone, watch them do a good job and get the right result, and then demote them on the back of it.

“It’s left a bad taste in the mouth for a lot of people after the high of the result at court.”

Rhys was shot and killed on August 22, 2007. The police team collected eight months of evidence against 18-year-old Croxteth Crew member Mercer and six others.

They were jailed for a total of nearly 40 years.

Mr Kelly was also in charge of the investigation into the murder of three- year-old Francesca Bimpson when he left the force.

He was also the first detective at the scene when toddler James Bulger was found dead.

A Merseyside police spokeswoman said: “Dave Kelly has decided to retire from the Force whilst still in the role of temporary Detective Superintendent.

“Dave Kelly decided to retire from the Force a couple of weeks ago, shortly before his temporary role was due to come to an end. As a result, he will retire on a substantive Superintendent's pension.

“Mr Kelly is a highly respected detective, and we wish him well for the future.”

The force added: “Merseyside Police does not appoint an officer to a higher rank without them going through the necessary application, recruitment and selection process.”