West Midlands police is failing to record more than 16,600 violent crimes each year and “failing domestic abuse victims”, according to a watchdog.

The force was given an overall rating of inadequate by Her Majesty’s Inspectorate of Constabulary and Fire and Rescue Services, which noted that about a third of staff had not received online training on how to record crimes.

The watchdog added that when the force does record crime reports, delays can result in victims not being referred to support services as soon as possible, which can affect the early support charities can provide.

Only 78.2% of violent crime and 89.2% of sexual offences reported were recorded, added the report.

In response, the force’s deputy chief constable, Louisa Rolfe, said it was frustrating that the force’s ranking had remained inadequate since 2017 “despite substantial progress”.

Rolfe said the report had focused only on violent crime and sexual offences and did not recognise overall crime, which she said the force dealt with with “good, reliable accuracy”.

Inspections for about 75% of police forces in England have been published for 2018, and of those those, two-thirds were graded as inadequate or requiring improvement.

The report said: “it remains a cause of concern that the force is still under-recording crimes relating to domestic abuse incidents and failing to give many of these victims a satisfactory service.”

In 2017, the watchdog said 38,800 crimes reported each year were not recorded by West Midlands police. Between 1 March and 31 May 2018, the force was re-examined for recording rates of violent crime and sexual offences. Inspectors were unable to look at other types of crime because the force was updating its systems.

Of the 2,176 reports of crime audited, 470 were related to domestic abuse. Of these, the force had recorded 354.

Out of the 116 offences not recorded, 95 were for violent offences, including common assaults, harassment, ABH and malicious communications.

One case study described a female caller who reported that her former partner was kicking the door of her home and making threats to assault her if she failed to open the door. The report said the offence should have been recorded as a crime and that the victim was a repeat victim of domestic abuse and the offender a known domestic violence offender.

Although officers attended the address, they failed to record the offence and did not provide an explanation why they thought a crime had not been committed.

The report said: “We found several examples of attending officers letting down victims by simply not believing them. Some incident logs contained closing comments that were completely different to the initial call and recorded no crime, without an adequate explanation.”

The force had not improved its crime recording for reports of rape since the 2017 inspection according to the report, which added it “remains a cause of concern”.

It found that only 144 of the 161 rape crimes examined had been correctly recorded. Of the unrecorded rapes, eight were not recorded at all and one was misclassified as a sexual assault.