Rapists are increasingly exploiting social media to cover their tracks and mislead investigators, a joint conference by police and prosecutors on rape was told on Wednesday .

At the police and the Crown Prosecution Service’s first joint initiative on rape, prosecutors said they had established an emerging pattern of behaviour where rapists constructed “false narratives” after the crime. One technique described involved rapists contacting victims the next day, sometimes by text or social media, thanking them for a sexual encounter. Defendants can try to rely on such messages should there be a trial.

Alison Saunders, the director of public prosecutions, told the event of the potential for social media to be used to “set up the scene” and warned that false messages could be used to set up a defence.

There has been a 30% increase in the number of rape cases coming to court in the past two years, partly as result of publicity over Jimmy Savile and historical abuse scandals but also because victims appear to have more confidence that their humiliating experiences will be believed.

Police and prosecutors at the event examined a hypothetical case where an 18-year-old student went to a freshers’ party with a more mature student in her first week at university, drank too much, took drugs then woke up to find he had come into her room.

Despite repeated requests for him to go away, he continued to take off her clothes. When it became clear he was going to rape her, she gave him a condom to give herself some protection. Asked whether they would prosecute the man for rape, an overwhelming number of hands in the room went up; not everyone at the conference was a prosecutor.

In the discussion that followed, officers and prosecutors suggested checking social networking sites should be standard practice – for gathering evidence and to find out whether defendants had posted comments that might be used later in a trial to put a deceptively innocent construction on an assault. “You could see how it ended,” one investigator suggested. “Did he stay the rest of the night? Did he send flowers?”

Martin Hewitt, a Metropolitan police assistant commissioner, who is the national lead officer on rapes and sexual assaults, told the conference: “The rise in reporting of rapes is something which I unreservedly welcome. The issue that causes me concern is the significant level of under-reporting. The reality is that we are still only getting 20-25% of those who suffer the offence … 75% of those [who are raped] are not coming forward.

“It should be more about fully investigating the suspect’s behaviour and building a strong case – making it less about the credibility of the victim and more about the credibility of the circumstances.”

Police and prosecutors were issued new guidance warning that “offenders may take steps which, on the face might seem normal or reasonable, to distance themselves from an offence or to reframe the offence … in order to undermine or pre-empt any allegation”.

Police were warned that offenders may try to contact their victim, ”being over-friendly or seeking reassurance” and “reinterpreting events leading up to the offence as spontaneous rather then planned”.

Other typical patterns of behaviour include boasting to friends about what happened, pretending to go to sleep immediately after an assault despite the obvious distress of the victim and making counter-allegations against the victim.

The conference was shown moving video interviews with survivors of rape, one sequence ending with the repeated refrain that: “I want to be believed.” One woman said: “I was the victim of a drug rape. I want the police to believe me and not to be blamed for drinking too much.”

Consent to sexual activity is not a grey area – in law it is clearly defined and must be given fully and freely Alison Saunders, director of public prosecutions

Hewitt said it should be understood that there were very few examples of false rape allegations. “The number of such cases is tiny. In cases of wrongful accusation, a person who has been accused has a right to fairness as anyone else does. It would not be legitimate to get to a place where we say this could not happen,” the assistant commissioner said. Saunders said: “For too long society has blamed rape victims for confusing the issue of consent – by drinking or dressing provocatively for example – but it is not they who are confused, it is society itself and we must challenge that. Consent to sexual activity is not a grey area – in law it is clearly defined and must be given fully and freely.

“It is not a crime to drink, but it is a crime for a rapist to target someone who is no longer capable of consenting to sex though drink. These tools take us well beyond the old saying ‘no means no’ – it is now well established that many rape victims freeze rather than fight as a protective and coping mechanism.

“We want police and prosecutors to make sure they ask in every case where consent is the issue: how did the suspect know the complainant was saying yes and doing so freely and knowingly?”

Under section 74 of the Sexual Offences Act 2003, consent is defined as being when anyone agrees to have sexual relations while having the freedom and capacity to make that choice.

Consent can be given to one sort of sexual activity but not another, or with conditions, such as wearing a condom. Consent can be withdrawn at any time during sexual activity and each time activity occurs.

In investigations detectives are expected to establish what steps, if any, a suspect took to obtain the complainant’s consent. The prosecution must prove that the suspect did not have a reasonable belief that the complainant was consenting.