Thames Valley Police failed to record 35,200 crimes in a year, the police watchdog has estimated.
Her Majesty's Inspectorate of Constabulary and Fire and Rescue Services rated the force as "inadequate" for the way it records reported crimes, in its latest inspection report, published in February 2018.
While the watchdog said Thames Valley Police had improved its recording methods, inspectors estimated that 20% of crimes which were reported to officers, 35,200 incidents, failed to be recorded.
These included reports of rape, domestic abuse and sexual offences.
The report said there is a failure of officers and staff to make correct crime recording decisions at the first opportunity.
“This is due to deficiencies in the force's crime recording processes, insufficient understanding of crime recording requirements and inadequate supervision,” it said.
“This means that the force is letting down many victims of crime.
“The force is failing to ensure it adequately records all reports of rape, other sexual and violence offences, including domestic abuse crimes.”
HMICFRS found that 69% of violent crimes reported were recorded, leaving 13,900 reported crimes unrecorded. The inspection also stated 90% of reported sex offences were recorded, with 490 incidents unrecorded.
The watchdog audited 1,970 crime reports between February and July 2017.
Among findings inspectors concluded 22 of 174 audited rape reports were misclassified or unrecorded.
However, they rated the force as "requires improvement" regarding the leadership and culture required to improve crime recording.
The report said: " The force needs to reinforce among officers and staff that all reports of crime must be recorded even in cases where the victim chooses not to support an investigation or prosecution."
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."