Three English police forces are failing to record thousands of reported crimes each year, new reports have found.
Thames Valley Police and North Yorkshire Police failed to record one in five crimes, Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS) found.
They are rated "inadequate" for recording crime.
Durham Constabulary was rated "good", but failed to record 8.5% of crimes.
HMICFRS has so far found 10 out of 20 forces in England and Wales "inadequate" in a series of audits of "data integrity".
A further five were told they require improvement.
Thames Valley was shown to have not recorded 35,000 crimes a year - about one in every five reported to it - including rape, violent assaults and domestic abuse.
North Yorkshire also failed to record 9,200 crimes, again about one in every five.
Durham did not record 4,700 crimes a year, HMICFRS said, including domestic abuse, public order and "young people sharing indecent images of themselves".
All three forces have said they will work to improve crime recording, although Thames Valley stressed there was no suggestion it had failed to respond properly to calls.
Rachel Almeida, from the charity Victim Support said not recording crimes could lead to victims being denied justice and could deter people from reporting crime.
She said: "It is extremely concerning that almost half of the police forces investigated have been found to be inadequate in recording crimes reported to them."
What is recorded crime?
If someone calls police about an incident, an officer may investigate and the victim may be given support.
However, the police still have to decide whether or not to "record" that incident as a crime under categories such as "theft" or sub-categories such as "assault without injury".
This data helps inform both the force itself in preventing, detecting and prosecuting crimes and it builds the national picture of whether certain types of crime are rising or falling.
Inspectors have looked at the data for 20 out of the 43 police forces covering England and Wales and found 15 of them need to be doing better to take those incidents they are told about and make sure they are properly recorded.
So far, Leicestershire has the biggest gap between crimes reported to it and those officially recorded, almost one in every four crimes.
Analysis by Daniel Wainwright, BBC England Data Unit
Thames Valley deputy chief constable John Campbell said the report made for "unwelcome reading".
He added: "The matters subject to review were recorded as 'incidents' rather than crimes, but there is no suggestion that we failed to respond appropriately to the calls or to deal with the matters in hand.
"I can assure the communities of Thames Valley that every day, every officer is working hard to keep you safe from harm and to protect victims of crime."
HMICFRS estimates almost one in five crimes in North Yorkshire are "not properly being recorded" - something it described as "inexcusable".
Flaws in the way the force recorded its crimes were identified by inspectors, with findings of a report saying there was "some confusion" as to who was taking responsibility for recording crimes.
North Yorkshire's Deputy Chief Constable Lisa Winward said the force "needs to do much better" when recording crimes and added they are working on the recommendations for improvement.
Durham's Police and Crime Commissioner Ron Hogg said: "Integrity around crime data recording is essential to maintaining public confidence and there is still some progress to be made."