The level of crime not recorded by Hampshire police was more than double a national average branded "indefensible" during a sample taken in an inspection.
The force was found not to record 40% of crime in a sample of incidents taken by HM Inspectorate of Constabulary, more than double the average of 19%.
The chief inspector of constabulary called the situation "indefensible".
The force said it was already changing the way it recorded crime but suggested this was partly because of cuts.
Det Supt Rachel Farrell said: "This has been necessary as we cut back office staff in order to keep as many officers as possible on the frontline.
"The HMIC audit has been timely. It comes part way through our change and highlights specific areas where we need to improve."
HMIC carried out an inspection of crime data in all police forces in England and Wales, during which it took a random sample of incidents and looked at which ones should be counted as crimes and how many the force had actually recorded.
It concluded that 112 incidents in Hampshire were crimes but only 67 had been recorded.
Chief Inspector of Constabulary Tom Winsor said: "The first duty of the police is to protect the public and reduce crime. A national crime-recording rate of 81% is inexcusably poor.
"Failure to properly record crime is indefensible. This is not about numbers and dry statistics; it's about victims and the protection of the public."
Hampshire Constabulary said some crimes, which previously would have been investigated first and then recorded, were now being recorded immediately.
It said it would also introduce new measures to build trust in reporting sexual offences.
However, Simon Hayes, Police and Crime Commissioner for Hampshire, said he was concerned at the under-recording of crime, especially for rape cases.
He said: "There has been a culture of 'investigate then record' by the constabulary, rather than 'record then investigate' as required by national guidance."