More than a quarter of sex offences - including rape - are not recorded as crimes because of "unacceptable failings" by police, a highly critical new report has found.
Under-recording of sexual offences was at 26% and the national rate of wrong decisions to cancel crime records for rape was 20%, Her Majesty's Inspectorate of Constabulary (HMIC) said.
Total under-recording of crime by all 43 police forces in England and Wales was found to be at an "inexcusably poor and indefensible" 19%, amounting to more than 800,000 crimes each year.
The inspection, which looked at over 8,000 reports of crime to the police between November 2012 and October 2013, discovered 37 cases of rape which were not recorded as crime.
And even when crimes were recorded correctly, many were removed or cancelled from the system as "no-crimes" - including 200 rapes and more than 250 violent crimes.
Her Majesty's Chief Inspector of Constabulary Tom Winsor said : "The position in the case of rape and other sexual offences is a matter of especially serious concern.
"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.
"The police should immediately institutionalise the presumption that the victim is to be believed.
"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.
"Victims need and are entitled to support and assistance. They - and their communities - are entitled to justice.
"Failures in crime-recording can also increase the risks to victims and the community of the denial of justice."
One in five of the 3,246 reviewed decisions to cancel a crime record as a "no-crime" were found to be incorrect.
Police are obliged to inform victims about their decisions, but in over 800 of the cases examined there was no record of the victim having been told
The police watchdog said that it had possibly given the impression to victims that crimes were being investigated when they were not.
The report also found that in more than a fifth of 3,700 cases, offenders were given out-of-court disposals such as a caution or a penalty notice when they should have been charged and sent to court or given a heavier punishment.
HMIC said that it had found "relatively little firm evidence" to show that undue pressure was being put on officers to manipulate the figures.
A survey of over 17,000 officers found that 39% of those with responsibility for making crime-reporting decisions admitted that concerns over performance and other pressures were distorting the picture.
And a poll of the public's views found that only 66% of people trust the police to record crime accurately.
Home Secretary Theresa May, who commissioned the report, said that it had found "utterly unacceptable failings" in crime recording
But she said matters were improving and pointed to a more than 20% drop in crime under the Coalition government.
Shadow policing minister Jack Dromey responded by calling for Mrs May to " get a grip on this and make urgent changes to the way the police record crime".
"This report is the latest blow to the Home Secretary's repeated claims that crime is falling, when the reality is that almost one fifth of crime is simply not being recorded," he said.
"It is time for Theresa May to come clean with the public and acknowledge that her crime stats just do not reflect reality."
Chief Constable Jeff Farrar, lead for crime recording at the Association of Chief Police Officers, said: "Police record over three million crimes against victims every year.
"It is important that those crimes are recorded accurately so that the police can take the right action for each victim of crime.
"Pressures from workload and target culture, use of professional judgment in the interests of victims, lack of understanding of recording rules or inadequate supervision can all lead to inaccurate crime recording.
"There have been allegations of improper practice, such as dishonest manipulation, in crime recording, however, the biggest and most in-depth inspection ever conducted by HMIC could not find anyone to come forward with any firm evidence to support this."
Chief Superintendent Irene Curtis, president of the Police Superintendents' Association, said that recorded crime was a measure of demand on police resources rather than police performance.
"Every victim of crime deserves to have that crime properly recorded and investigated, and I support HMIC's position that crime recording should be focused on the victim," she said.
"In the majority of cases this is already the approach that forces take but it should happen every single time.
"HMIC's report covers a period of at least 12 months ago and recognises that considerable improvements have already been made since that period.
"The focus on crime recording that HMIC's inspections have provided, together with recommendations for training to further strengthen leadership, should see these improvements continue.
"HMIC also acknowledges the impact that the historic performance culture has had on recorded crime and that it takes time to change such a culture.
"Many forces are moving away from this culture, however I believe that we will not fully achieve this until it is universally acknowledged that recorded crime is actually an indicator of demand on police resources and not a measure of police performance."