Almost half a million preyed upon every year according to the first joint overview of sexual offending in England and Wales

Nearly one in five of all women in England and Wales report that they have been the victim of a sexual offence since the age of 16, according to a new official analysis.

The study says there are about 473,000 adult victims of sex crimes every year and, while most involve unwanted touching and indecent exposure, they include 60,000 to 95,000 victims of rape.

The first joint statistical overview of sexual offending in England and Wales by the Ministry of Justice, Home Office and Office of National Statistics confirms that only a very small proportion of sexual offences lead to a conviction. In the last three years court statistics show an average of 5,620 offenders convicted each year for all types of sexual offences, with 1,070 convicted for rape.

The estimate that there are 473,000 victims of sexual offences every year, of whom 400,000 are women, is drawn from the Crime Survey for England and Wales. It estimates that there were 97,000 victims of the most serious sexual offences last year, of which there were around 69,000 female victims of rape. The analysis adds that 90% of such victims knew the perpetrator.

But the official statisticians say they no longer regard as accurate widely quoted previous estimates that only 6% of allegations of rape reported to the police result in a conviction for rape. "They were ballpark estimates of what the likely figure is which we no longer regard as accurate," said one. The statisticians say that they are in the early stages of establishing a more authoritative figure but face severe difficulties in comparing victim surveys with recorded offences and the number of convictions of offenders.

The new official analysis follows an inquiry into rape reporting by Lady Stern which concluded that a misleading focus on the conviction rate left victims' needs neglected and stopped women coming forward. Campaigners such as Women Against Rape criticised this, saying it let the criminal justice system off the hook while women were still receiving "shocking" treatment.

The new analysis confirms that on a more usual definition of conviction rate – the proportion of cases prosecuted in the courts – 62.5% of rape cases ended in a conviction, eight points higher than in 2005. The 1,200 convictions for rape in 2011 compare with 800 in 2005.

It also shows that 95% of convicted rapists are jailed, and for increasingly long sentences. In 2011, the average sentence for rape was eight years and six months, 20 months longer than in 2005. The 19 rapists who received a caution in 2011 included 16 under 18 years old.

The statistical analysis does show that rape and other sexual offences remain under-reported to the police compared with many other crimes. Only 15% of women said they reported the offence to the police. Their reasons for not going to the police included "embarrassing", "didn't think the police could do much to help", "too trivial/not worth reporting", and "private/family matter".

The study also confirms that it takes far longer to complete a rape case than nearly all other criminal cases, taking an average of 675 days from the date of the offence to completion of the case in 2011. This compares with 162 days for cases of violence against the person and an average of 154 days for all criminal cases.

Martin Hewitt of the Association of Chief Police Officers said police had been working hard to improve recording and reporting practices to encourage victims to report sexual offences.

• This article was amended on 11 January 2013. The original headline referred to sexual assault, rather than sexual offences. This has been corrected.