An HM Inspectorate of Constabulary (HMIC) report has exposed the police for failing to record more than a quarter of sex offences in England and Wales.
According to the watchdog's report, the failure was "indefensible", as more than 800,000 of all reported crimes went unrecorded annually.
"The position in the case of rape and other sexual offences is a matter of especially serious concern," said Chief Inspector of Constabulary Tom Winsor.
"It is particularly important that in cases as serious as rape, these shortcomings are put right as a matter of the greatest urgency. In some forces, action is already being taken in this respect."
"If evidence later comes to light which shows that no crime occurred, then the record should be corrected; that is how the system is supposed to work."
The report raked through more than 8,000 reports of crime across all 43 police forces in England and Wales between November 2012 and October 2013.
However, the watchdog found that there were some major discrepancies.
Some 37 cases of rape that were not recorded as a crime while 20% if the 3,246 no-crime decisions were found to be incorrect.
Even when crimes were recorded correctly, the watchdog found that police had removed or cancelled the incidences from the system as "no-crimes", which occurs when something is reported to the police but not recorded as an offence.
Home Secretary Theresa May said: "It is never acceptable for the police to mis-record crime. Failing to do so not only lets down victims, but the wider public who expect to be able to trust the integrity of police recorded crime."
Police representatives said the situation had improved since the study.
"HMIC's report covers a period of at least 12 months ago and recognises that considerable improvements have already been made since that period," said Chief Superintendent Irene Curtis, president of the Police Superintendents' Association.