The number of people reporting being raped on their first date with someone they met on a dating app has risen six-fold in five years, figures show.
There were 184 rape allegations in the UK in 2014, up from 33 in 2009, the National Crime Agency (NCA) said.
It said 85% of the victims of sexual offences linked to online dating were female in the period 2003-15.
The agency said it was concerned the number may be under-reported, and it encouraged victims to come forward.
More than nine million people in the UK have signed up to dating websites.
'Make people aware'
The overall number of rapes recorded in England and Wales similarly rose between 2009 and 2014, but far less sharply, figures for the Office for National Statistics suggest.
Sean Sutton of the NCA's Serious Crime Analysis Section said further work was needed to understand the increase, but the team was considering whether this could include:
- The fact that people feel protected online, and their communication can escalate rapidly to become sexual in nature, leading to mismatched expectations
- Deliberate targeting of online sites and apps by those who intend to commit sexual assault
- Victims having more confidence to report assaults to the police
- Whether more people are spending time in private on a first date
There was some evidence of coercion and persuasion being used by offenders to encourage, often reluctant, victims to meet sooner than they would like, Mr Sutton said.
He said data from 2003-15 suggested 43% of first face-to-face meetings between a victim and offender took place within one week of their initial contact online.
"This initial work clearly raises a lot of questions and we will be working with academia to build a more complete picture," Mr Sutton said.
"Our aim here is to make people aware of the potential danger, so they can be better prepared and make the choices that are right for them.
"A rape victim is never at fault and we do not want the circumstances in which these assaults take place to cause any victim to doubt that.
"Sexual assault is a crime, full stop, and we want victims to feel confident reporting it to the police."
Online dating safety advice
1. Plan it. Say it. Do it - It is your date. Agree on what you both want from it before you meet up. Do not feel pressured to meet before you are ready or for any longer than you are comfortable with - a short first date is fine.
2. Meet in public - Stay in public. The safest plan is to meet somewhere public and stay somewhere public. Make your own way there and back and do not feel pressured to go home with your date. If you feel ready to move to a private environment, make sure your expectations match your date's.
3. Get to know the person, not the profile - The way people interact online is not always the same face-to-face. Do not be offended if your date is more guarded when meeting in person or if things do not progress as fast face-to-face.
4. Not going well? Make your excuses and leave - Do not feel bad about cutting a date short if you are not keen. You do not owe the other person anything, no matter how long you have been chatting or what has been suggested.
5. If you are raped or sexually assaulted on your date, help is available - Contact Rape Crisis or The Survivors Trust for more information and advice.
Source: Get Safe Online
George Kidd, chief executive of the Online Dating Association, (ODA) which represents online dating businesses and which is supporting the campaign, said: "Sexual assault or abuse is never acceptable.
"Even one incident of harm is one too many.
"The NCA is right to look at what happens online and the ODA are already working with them in getting the right messages out to people."
Katie Russell, spokeswoman for Rape Crisis England and Wales, said the report showed that dating websites and apps were now established among the range of means sexual offenders use to target and access women and girls.
"Attention clearly needs to be focused on targeting the minority who abuse dating services to perpetrate rape and other forms of sexual violence," she said.
Earlier this month, security firm Symantec warned that almost half of people using dating sites or apps had been scammed or spammed.
It said daters fell victim to blackmail, were subjected to revenge porn and were tricked by people who assumed fake identities to steal cash.
Correction 17 February 2016: This report has been amended to clarify that figures for the proportion of victims being female and the proportion of first face-to-face meetings between victim and offender taking place within a week of first online contact both apply to the period 2003-15.