A fifth of crimes - equivalent to tens of thousands of offences - could be going unrecorded by police, a damning report has found.
An inspection of 13 forces found 14 rapes were among offences not recorded by officers, including an allegation made by a 13-year-old autistic boy written off as "sexual experimentation".
Another rape was not recorded due to "workload pressure" as recording the crime would "entail too much work", the report by Her Majesty's Inspectorate of Constabulary (HMIC) said.
The police watchdog also found some offenders have been issued with out-of-court disposals, such as cautions, when they should have been prosecuted.
And inspectors said they could not rule out " discreditable or unethical behaviour" on the part of officers for the failure rate.
Home Secretary Theresa May said the report exposed "unacceptable failings by the police" and warned that once HMIC concludes its work in October, official figures may show a spike in police recorded crime.
HM chief inspector of constabulary Tom Winsor said: "The consequences of under-recording of crime are serious, and may mean victims and the community are failed because crimes are not investigated, the levels of crime will be wrongly under-stated, and police chiefs will lack the information they need to make sound decisions on the deployment of their resources."
The police watchdog is conducting an inspection into the way all 43 forces in England and Wales record crime data and said if its findings so far reflect the national picture, it could mean 20% of crimes may be going unrecorded.
Figures from the Office for National Statistics (ONS) last week showed police recorded 3.7 million offences in the year to December 2013 - but if HMIC is correct, the real figure could be as high as 4.4 million.
Out of a sample of 3,102 incidents, HMIC found 2,551 crimes should have been recorded but 523 were not, including sexual offences, crimes of violence, robbery and burglary.
In one example given by the watchdog within an unidentified force, a 13-year-old child with autism told his parents that he had been sexually assaulted by a 15-year-old male friend.
Police were contacted but n o crime was recorded on the grounds that to do so would have a negative effect on the victim and it was wrongly written off as sexual experimentation, HMIC said.
Another example where "workload pressure" was given as the basis for not recording a crime was a report of rape, according to the findings.
The report said: "In this example, it was considered that recording the crime would entail too much work, as the officer made a judgment that the circumstances of the complaint made it unlikely that the case would be prosecuted."
HMIC also found that more than one in 10 out-of-court disposals reviewed, which included cautions, penalty notices for disorder (PNDs), cannabis warnings and community resolutions, should not have been due to the offending history of the individual. In some of these cases, the offender should have been charged.
Turning to motive, inspectors said: "I n the light of what we have so far found, which could conceptually be contradicted by later results, it is difficult to conclude that none of these failures was the result of discreditable or unethical behaviour. The failure rate is too high."
Police forces inspected so far are Cheshire, City of London, Devon and Cornwall, Essex, Gloucestershire, Greater Manchester, Gwent, Hertfordshire, the Metropolitan Police, Norfolk, North Wales, North Yorkshire and South Yorkshire.
The Home Secretary said: " HMIC's interim report exposes unacceptable failings by the police.
"It is quite possible, once HMIC has completed its work on recorded crime statistics and made recommendations on how the police need to improve, that we will see an increase in recorded crime.
"If that increase is driven by improved accuracy in the recording of crime or more victims reporting crime to the police, we should welcome it.
"Such an increase would not mean that crime itself is rising."
She added: "Despite these concerns about the accuracy of recorded crime statistics, we can be confident that crime is falling and is at its lowest level since the crime survey began in 1981."
The shocking report comes after serious concerns were raised over the integrity of crime figures, sparked by claims made by former Metropolitan Police officer James Patrick last year.
Mr Patrick, who has since resigned, told MPs that massaging crime figures to hit performance targets had become ''an ingrained part of policing culture''.
His comments, combined with further evidence submitted to Parliament, ultimately led to the UK Statistics Authority stripping police-recorded crime figures of their gold-standard status.
Adam Pemberton, assistant chief executive at charity Victim Support, said: " This is about much more than inaccurate statistics or poor number-crunching - each mistake represents a victim losing their chance to get justice and to access support services.
"It is completely unacceptable that victims of any crimes - let alone serious sexual offences such as rape - should have their complaints go unrecorded or downgraded because of police incompetence or even laziness.
"We know from speaking to tens of thousands of victims of crime every year how important it is victims get the help they need and that they trust the police to carry out a thorough investigation.
"Most victims want more than anything for their offender not to commit another crime - but if the police can't reliably recognise and record when and where a crime has been committed, their efforts to cut crime may well be misdirected."
Shadow home secretary Yvette Cooper said: " Though only an interim report, the Inspectorate's findings are extremely serious."
National police lead for crime statistics, Chief Constable Jeff Farrar, said: "The vast majority of officers join the police motivated by a desire to protect the public; nobody joins the police service with the intention of recording crime inaccurately.
"However, it is clear that the service has not always met the standards of data quality that the public expects.
"The service is already actively addressing many of the weaknesses identified by the report: we're reviewing the way we record sexual offences across the country and we're working with the College of Policing to implement the Code of Ethics, which highlights the need for ethical recording."
Bernard Jenkin, the Tory MP who chairs the Commons public administration select committee (PASC), said the "devastating" HMIC analysis corroborated his committee's own findings.
"Like PASC, HMIC is 'seriously concerned' about 'weak or absent management and supervision of crime recording, significant under-recording of crime, and serious sexual offences not being recorded'," he said.
"They are finding cases when the police have failed to prosecute offences when they should have done. This both under-records crime and over-states clear-up rates.
"Our key witness, whistle-blower PC James Patrick, said crime was under-recorded by as much as 20%.
"HMIC says the same. They also accept that it is 'difficult to conclude that none of these failures was the result of discreditable or unethical behaviour'.
"This is why PASC recommended that the Committee on Standards in Public Life should study the values and ethics of police leadership."
Mr Winsor said the report found "serious weaknesses" and it was impossible to rule out unethical behaviour by officers.
He told BBC Radio 4's Today programme: "We believe that it is weak or absent management, poor knowledge on the part of those making the crime recording decisions or the crime recording rules, weak or absent training and workload pressures.
"But because the failure rate is as high as it is - perhaps as high as 20%, we'll know in October - then it is impossible to rule out discreditable or unethical behaviour."
Gwent Police Chief Constable Jeff Farrar, who speaks for the Association of Chief Police Officers on crime statistics, told Today: "There are cases of a lack of knowledge, and in some forces perhaps we could benefit from greater training.
"There is clear evidence that some people have neglected their duty and I know my chief constable colleagues, who I have spoken to in the last couple of days, are waiting for their finalised reports so they can look into those in more detail and deal with those incidents."
He said that he was not familiar with the details of the unrecorded rapes, but added: "There does appear to be serious failings there and there does appear to be serious concerns."