Thousands of reported crimes including rape and domestic abuse are going unrecorded by police, a watchdog has warned.

The shortcomings mean victims are being failed, inspectors said. Two forces – Thames Valley and North Yorkshire – were singled out for their “inadequate” crime recording by Her Majesty’s Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS).

The assessment of Thames Valley found that nearly one in five offences (19.6%) were not being properly recorded, which equates to approximately 35,200 crimes a year.

Crimes that are going unrecorded by the force include sexual offences, domestic abuse and rape.

Zoe Billingham, the inspector of the constabulary, said she was “disappointed” with the quality of crime recording in Thames Valley, but that she was satisfied the force works hard to ensure victims of crime are safeguarded.

She said: “It now needs to ensure that it records crimes at the earliest opportunity, and that there is proper supervision of crime-recording decisions.”

The report on North Yorkshire estimated that the force fails to record 9,200 reported crimes a year, also including sexual offences, domestic abuse and rape.

North Yorkshire’s inspector of constabulary, Matt Parr, said the police had implemented most of the recommendations from their 2014 report, adding they still found plenty of room for improvement.

He said: “As it stands today, we estimate almost one in five crimes in North Yorkshire are not properly recorded. This is simply inexcusable.

“The force has robust processes in place to ensure the safeguarding of victims of these crimes, but too many offences continue to go unrecorded and therefore not investigated properly.”

He added that North Yorkshire police were “potentially depriving victims of the services and justice to which they are entitled”.

Thames Valley police said they accepted the findings and will be addressing the concerns raised.

John Campbell, the force’s deputy chief constable, said: “This reports makes for unwelcome reading especially for a force that performs so well when measured in terms of our effectiveness, efficiency and legitimacy.”

He added he was pleased the watchdog confirmed they found no issues of unethical behaviour.

“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,” he said.

Lisa Winward, the deputy chief constable of North Yorkshire police, acknowledged the force needs to do “much better”.

She said: “Based on its inspection last year, HMICFRS found that although our officers and staff are focused on the needs of victims, our administration is letting us down, and we are not recording all crimes as we should. That must change, and we have already started to make improvements.”

The findings are the latest from a series of rolling inspections looking at the crime data “integrity” of every police force in England and Wales.

Inspectors launched the programme in November 2015 after finding the national average of under-recording of crimes stood at an “inexcusably poor” 19%.