Police are failing to properly record tens of thousands of offences, including rape and violent crimes, according to inspectors.
HM Inspectorate of Constabulary (HMIC) assessed four forces' crime recording.
Merseyside and Devon and Cornwall were both rated "inadequate" after inspectors found they were under-reporting serious crimes including violent and sexual offences.
Inspectors said Northumbria and Avon and Somerset "required improvement".
HMIC said Merseyside Police recorded only about 84% of crimes reported to it and failed to record an estimated 19,200 reported crimes each year.
Inspector of Constabulary Mike Cunningham said crime recording processes within the Merseyside force were "not fully effective".
Merseyside was under-recording too many reports of violent and sexual offences, excluding rape, the report said.
It added: "Those failings are depriving many victims of the services to which they are entitled and are a cause of concern."
Deputy Chief Constable Carl Foulkes, of Merseyside Police, said the force had already implemented some recommendations and was "working to rectify other areas raised".
Devon and Cornwall Police was also rated "inadequate", after inspectors found it was only recording 82% of crimes, amounting to more than 17,400 crimes not being recorded.
Estimated number of crimes not being recorded each year
- Merseyside - 19,200
- Devon and Cornwall - 17,400
- Avon and Somerset - 13,700
- Northumbria - 7,300
"I was most concerned to find that the [Devon and Cornwall] force had failed to record reports of rape, serious sexual assault and offences of serious assault and human trafficking," Wendy Williams, another inspector from the watchdog, said.
She said victims were not getting enough service when they initially reported crime.
Devon and Cornwall Police accepted it had to put better processes in place to accurately record all crime.
"We don't believe that in the vast majority of circumstances we have not supported a victim," Deputy Chief Constable James Vaughan said.
"This is more about recording a crime, helping a victim, but then not properly updating systems around supplementary crimes related to the same investigation."
The Avon and Somerset and Northumbria forces were both judged as "requiring improvement" after they were found to be recording about 90% and 93% of crimes reported to them respectively.
They were both criticised for under-recording serious offences such as rape.
Both forces said they recognised that, while they had made improvements since inspections in 2014, "there was still more work to be done".
The forces are the latest to come under scrutiny as part of rolling inspections looking at the crime data integrity of every police force in England and Wales.