One of the biggest police forces in the country fails to record more than 38,000 reported crimes each year, including a quarter of violent offences.
Greater Manchester Police (GMP) was graded "inadequate" at recording crime, and a watchdog found officers were also wrongly cancelling recorded violence, robbery and sex offences.
Her Majesty's Inspectorate of Constabulary (HMIC) said GMP recorded around 85% of crimes that were reported, but the force was under-recording some serious offences. A quarter of violent crimes, equivalent to more than 16,800 offences, went unrecorded in a year.
Dru Sharpling from HMIC said: "Despite making some progress following our 2014 inspection, the force is failing some victims of crime.
"We estimate that the force fails to record over 38,000 reported crimes each year. The reported crimes that go unrecorded include serious crimes, such as violence and sexual offences.
"The failings are often a consequence of a lack of knowledge on the part of the officers and staff as to their responsibilities for crime-recording; including the cancellation of recorded crime records."
In dip samples of cancelled recorded crimes, 18 out of 20 rapes were found to have been correctly dropped, and 17 out of 21 other sexual offences.
But only 10 out of 20 violent crimes and 15 out of 22 robberies that were audited had been correctly cancelled.
GMP said recording levels had risen from 68% to 85% in the past two years, and that further progress would be made once a new IT system was introduced.
Deputy Chief Constable Ian Pilling said: "Many victims of crime are satisfied with the service they receive, even when the crime is not recorded properly and the report doesn't highlight this.
"Whilst there are some unacceptable crime recording failings, many are simply administrative issues and do not mean we have failed the victim.
"A significant amount of activity has taken place to address these administrative problems and we will continue to work hard to address this."
He also stressed that the majority of unrecorded violent crimes were "in the less serious categories".
Another force, Staffordshire Police, was graded as "requires improvement" when recording crime.
HMIC said that 91% of reported crimes were recorded, including every rape, but that the force was under-recording offences including violence, sex offences and modern slavery.
Wendy Williams from the watchdog said: "The force is still not recording a large number of crimes each year properly - approximately 6,700 crimes, including some serious crimes, such as violence and sexual offences.
"There is a lack of knowledge amongst officers and staff about their responsibilities to record crime."
Meanwhile, Sussex Police was judged to be "good" at recording crime, with a rate of nearly 95%, although 5,300 offences were going unrecorded, including some serious allegations.
Chief Constable Giles York said: "We were inspected at no notice by the HMIC and to have achieved 95% accuracy is a huge testament to how our staff understand what this means for victims of crime.
"Some victims may not have had their crimes recorded entirely accurately and if they have felt let down by that, then I am really sorry.
"We will need to keep working hard to maintain and improve this very high standard for recording crime to ensure that victims get the services they need."