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Kent Police has been rated inadequate in recording crimes and failed to record more than 24,000 offences, including rape and other sexual offences.
That means around 16% of all reported crimes went unrecorded between June and November last year.
Her Majesty's Inspectorate of Constabulary (HMIC) found the force was failing many victims of crime.
It has now ordered Kent Police to urgently implement its recommendations.
It said: "The serious causes of concern found during this inspection are such that HMIC may re-visit the force in early 2018 to assess progress."
Improvements ordered by inspectors after a visit in 2014 had been made, but not all had been maintained since, it said.
This regression undermined the effectiveness and efficiency of the force's crime-recording requirements.
Reports of crime made directly to public protection teams were often not being recorded, it found.
On top of this, crime-recording accuracy was not being scrutinised, and there had only been limited progress in ensuring officers and staff understand their crime-recording responsibilities.
It was found Kent Police officers and staff were not making the correct crime-recording decisions far too often.
The force control room also came in for criticism for the way it manages incidents; there was insufficient understanding of crime-recording requirements, and limited supervision to correct decisions made by officers and staff.
In fact, inspectors found the force failed to ensure it adequately records all reports of rape, other sexual offences and violence, including domestic abuse crimes and crimes reported directly to its public protection units.
The watchdog found in all 83.6% of reported crimes were recorded but that meant more than 24,300 crimes - or 16.4% - were not reported each year.
Chief Constable of Kent Alan Pughsley said: "Kent Police’s Crime Data Integrity Report makes for disappointing reading, however we accept its findings and have responded quickly to them to make the changes we need to improve the accuracy of our crime recording.
"There are a number of instances where although we have not recorded a crime, we have responded to the victim’s needs, conducted an effective investigation and provided safeguarding to the victims.
"All of those crimes that were not recorded have been thoroughly reviewed and safeguarding has been put in place.
"I want to be clear that victims are always at the heart of what we do – something that Her Majesty’s Inspectorate of Constabulary recognised in its findings. Our officers and staff approach crime recording in a positive and ethical way and we have no desire to under-record crime. '
"I would like to apologise to any victim who has not received the service they were entitled to.
"I would like to apologise to any victim who has not received the service they were entitled to" - Chief Constable Alan Pughsley
"However, on reviewing the recording decisions from this last year, it is apparent there have been administrative errors in the way we have been recording crime which has not been in line with the guidelines.
"We have now identified this and are working closely with HMIC to ensure all of the issues that led to those crimes not being recorded in the last year are reviewed so that our accuracy level increases. Extensive training is underway for the most up to date criteria in decision making around recording crime.
"Since the inspection, we have worked hard with HMIC to improve our crime data integrity, not just for the last year, but going forward, and will not rest until we are satisfied it is the best it can be."
On a positive note, the Crime Data Integrity inspection report found officers and staff are placing the victim at the forefront of their crime-recording decisions, and the force's victim support hub is well established.