A police force "fails to record over 38,000 reported crimes each year", a report has estimated.
Greater Manchester Police (GMP) was "under-recording too many reports of crime" including rape, violent crimes and sexual offences, HM Inspectorate of Constabulary (HMIC) said.
The report found there was a "systemic failure of officers and staff".
GMP Deputy Chief Constable Ian Pilling said the force needed to improve but it had made progress.
The assessment, completed in May, rated crime recording "inadequate" and estimated that 38,000 reported crimes each year were not recorded, based on an examination between 1 September 2015 and 29 February 2016.
The inspectorate found that GMP:
- Recorded about 85% of crimes reported to it
- Incorrectly cancelled recorded sexual offences (excluding rape) and offences of robbery and violence
- Had "limited supervision to support officers and staff in making good and prompt crime-recording decisions"
- Had a lack of understanding among officers and staff of their responsibilities for crime-recording
- Had "made some progress" in putting the victim at the centre of its crime-recording decisions and had "made good progress" against a national plan to improve crime-recording
Inspectors said the force had made "some progress" since an inspection in 2014 but "the force is failing some victims of crime".
It added: "There is a systemic failure of officers and staff to make correct crime-recording decisions at the first opportunity.
"This is due to the force often failing to provide a timely response to a report of a crime, not understanding crime-recording requirements, and limited supervision to correct these decisions and improve standards from the outset.
"The force is not always recording reported violent crimes, sexual offences and crimes reported to its public protection investigation units. This means that the force is failing victims of crime too frequently."
The inspectorate said it wanted improvements made within six months.
New IT system
Mr Pilling said: "Whilst there are some unacceptable crime recording failings, many are simply administrative issues and do not mean we have failed the victim."
He said most of the failures concerning violent crime "were in the less serious categories".
"It is important to recognise that there is nothing in the report to suggest that the integrity of officers and staff is in question."
Tony Lloyd, interim mayor of Greater Manchester and the region's police and crime commissioner, said he was making "major investment", as "one of the most significant reasons for GMP's poor performance is inadequate IT systems".