Police are failing to properly record tens of thousands of offences including rape and violence, a watchdog has warned.
Inspectors raised concerns after assessing the accuracy of crime recording at four forces - Merseyside, Northumbria, Devon and Cornwall, and Avon and Somerset.
HM Inspectorate of Constabulary (HMIC) judged Merseyside Police as "inadequate" after finding that it records only around 84% of crimes reported to it.
It estimated that the force fails to record more than 19,200 reported crimes each year.
Merseyside was under-recording too many reports of crime including violent and sexual offences, but 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."
HM Inspector of Constabulary Mike Cunningham said it was clear from the inspection that crime recording processes within the force are "not fully effective".
Devon and Cornwall Police was also rated as "inadequate" on the issue, with HMIC finding that it records around 82% of reported offences.
The watchdog estimates therefore that the constabulary is failing to record more than 17,400 reported crimes each year.
HM Inspector of Constabulary Wendy Williams said: "I was most concerned to find that the force had failed to record reports of rape, serious sexual assault and offences of serious assault and human trafficking."
She said victims in the force area are not receiving a good enough service when they initially report crime.
Avon and Somerset and Northumbria forces were found to be recording around 90% and 93% of crimes reported to them respectively.
Both were judged as "requiring improvement" in the area.
Inspectors estimated that more than 13,700 reports of crime are not being recorded by Avon and Somerset each year.
The force is under-recording serious offences such as rape and modern slavery crimes, HMIC found.
Northumbria Police is estimated to be failing to record 7,300 crimes annually, including violent offences and rape, the watchdog said.
Processes within the force were said to be "deficient" - leading to errors in how crimes are recorded.
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.
HMIC announced the programme in November 2015 after finding the national average of under-recording of crimes stood at an "inexcusably poor" 19%.
Shadow Home Secretary Diane Abbott described the latest reports as "deeply troubling", adding: "The public will be very concerned that serious crime including violent crime and rape could go so widely under recorded."
Deputy Chief Constable Carl Foulkes, of Merseyside Police, said: "The force has worked hard to improve its crime recording, but the HMIC has identified some areas of concern and we acknowledge that when we get it wrong we need to hold our hands up and look at what went wrong, why it went wrong and how we can ensure that we learn from the mistakes made and lessen the chances of it happening again.
"Already we have been able to implement some of the recommendations that the HMIC put forward and work is ongoing to rectify the other areas raised."
Deputy Chief Constable James Vaughan, of Devon and Cornwall Police, said: "I accept we and other forces have got to ensure we have 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.
"This is more about recording a crime, helping a victim, but then not properly updating systems around supplementary crimes related to the same investigation."
Deputy Chief Constable Winton Keenan, of Northumbria Police, said the force "takes the accurate recording of crime very seriously" and the force has made "significant improvements" since the last inspection in 2014.
He added: "Despite the improvements we have made, we recognise there is still more work to be done."
Nikki Watson, Assistant Chief Constable at Avon and Somerset Police, said: "Since our last inspection in 2014 we've made great strides in improving our crime recording accuracy, but there's still more work to be done.
"The inspectors identified areas for improvement and we welcome their recommendations."