Around 700,000 people were sexually assaulted in the year up to March, with the number recorded by police tripling in recent years.
Women accounted for 560,000 of all victims, while 140,000 were men.
The rise “largely reflects” improved police records and more victims being willing to report, according to the Office for National Statistics (ONS).
But most cases do not see criminal justice – less than one in five victims reported an assault and many of the offences that were told to police did not lead to a conviction.
Rape was the offence least likely to result in the offender being charged or sent to court, with just 3% out of the 54,000 rape offences in the year to April seeing this outcome.
“This lower rate for rape offences may reflect that these crimes can be difficult to investigate due to a lack of corroborating evidence or, in cases concerning two adults, where there are complex decisions to be made around consent and one person’s word against another,” the ONS said.
“Evidential difficulties”, such as police being unable to build a strong enough case or the victim or offender not being traced, meant just over half of all sexual assault cases didn’t make it beyond police investigation.
Resource pressures on forces led in part to a fall in the proportion of cases resulting in a charge or summons (6%, down from 13% in 2016), with an increase in complex and non-recent cases taking up more time to investigate.
“An increase in digital evidence in recent years has increased the complexity of evidence gathering, and non-recent cases may take longer to investigate before an outcome can be assigned,” the ONS said.
This is echoed by a 10% fall in defendants proceeded against at a magistrates court after being charged.
Alexa Bradley, from the ONS centre for crime and justice, said: “We have brought together data from different sources to show the path of sexual offences through the criminal justice system. Looking at these together, it’s apparent that the majority of victims don’t report the offence to the police.
“The report highlights that investigating sexual offences is challenging. Many offences don’t proceed further than the police investigation due to evidential difficulties. In addition, investigation are becoming more complex due to an increase from phones, tablets, computers and social media.
“We hope that providing insights into this serious issue will assist all those working to achieve better outcomes for victims.”
In nearly a third of offences, the victim did not support further action, with fear of being judged and not wanting to take action against an offending partner were among the reasons cited.
The figures were brought together from the ONS in collaboration with the Home Office, Ministry of Justice, and the Crown Prosecution Service.
Data covered young people and adults aged 16 to 59.
Useful helplines and websites:
- Victim Support - Visit victimsupport.org.uk or call 0808 168 9111 Sexual Abuse Referral Centres - Find a SARC
- Rape Crisis - Visit rapecrisis.org.uk or call 0808 802 9999 The Rape and Abuse Line - Visit rapeandabuseline.co.uk or call 0808 800 0123 (answered by women) or 0808 800 0122 (answered by men).