Hertfordshire Constabulary failed to record 11,200 crimes in a year, the police watchdog has estimated.
Her Majesty's Inspectorate of Constabulary and Fire and Rescue Services rated the force as "requires improvement" for the way it records reported crimes, in its latest inspection report, published in November 2017.
While the watchdog said Hertfordshire Constabulary had improved its recording methods, inspectors estimated that 13% of crimes which were reported to officers, 11,200 incidents, failed to be recorded.
These included reports of sexual assault, harassment and malicious communications.
The report said the force had made a concerted effort to record crime more accurately since the previous inspection in 2014.
“Work remains to be done, however. Officers and staff too often fail to make correct crime recording decisions at the first opportunity when dealing with reports of violent crime, including cases of domestic abuse,” it said.
“This is due to deficiencies in the constabulary's crime recording processes, insufficient understanding of crime recording requirements, and limited supervision.
“This means that the constabulary is letting down too many victims of crime.”
HMICFRS found that 85% of violent crimes reported were recorded, leaving 3,700 reported crimes unrecorded. The inspection also stated 96% of reported sex offences were recorded, with 70 incidents unrecorded.
The watchdog audited 1,215 crime reports between December 2016 and March 2017.
Among findings inspectors concluded four of 79 audited rape reports were misclassified or unrecorded.
However, they rated the force as "outstanding" regarding the leadership and culture required to improve crime recording.
The report said: "Senior officers demonstrate strong leadership with regard to crime recording expectations."
Chief Constable Bill Skelly, National Police Chiefs' Council lead for crime recording, said: "Understanding crime in our communities is a core task of policing.
"It helps inform prevention activity and the identification of victims, witnesses and offenders and the services that forces and partners have in place."
Mr Skelly explained that police recording is one of many sources which forces use to develop an understanding of crime in their area.
"The NPCC continues to work closely with HMICFRS, the Home Office and forces to provide guidance to officers and staff on a proportionate application of the counting rules."