Victims of crimes, including those alleging rape, are asked to hand their phones over the shelf – or risk prosecutions not going ahead.
Consent forms asking for permission to access information including e-mails, messages and photographs, have been rolled out in England and Wales.
It comes after a number of rape and serious sexual assault cases.
Victim Support said the move could stop victims from coming forward.
- Director of public prosecutions criticized over case failures
- Hundreds of cases dropped over disclosure failings
But police and prosecutors say they are trying to plug in the law that says complaints and witnesses cannot be forced to disclose phones, laptops, tablets or smart watches.
Director of Public Prosecutions Max Hill said they would only be looking at where they can find the "reasonable line of inquiry", with only relevant material going before and if it meets stringent rules.
The digital consent forms can be used for complaints in any criminal investigations, but they are most likely to be used in rape and sexual assault cases, where often know the suspect.
The forms state that victims will be given the chance to explain why they do not want to give consent to the police to access the data, but they are also told: "If you refuse permission for the investigator the material which would enable the defendant to have a fair trial then it may not be possible for the investigation or prosecution to continue. "
By Clive Coleman, BBC legal correspondent
Asking victims, complainants and witnesses – including those alleging rape – to agree on having their smartphones and mobile devices examined is a big ask.
Most modern phones have more computing power than that which Nasa missions.
They contain photographs (sometimes intimate), emails and social media postings – (sometimes deeply personal, sometimes indiscreet) – not to mention drunken late-night texts.
Many people guard the contents of their smartphones jealously and would regard the police examination as an invasion of privacy.
Civil liberties groups have raised concerns that may not report crimes if they fear their smartphone will need to be examined.
However, the shelves do not have the power simply to seize the phones of victims and witnesses, so consent is the only option. Will people willingly hand over their devices? Would you?
The issue of disclosure came to the fore when several court cases were halted because of evidence not being shared with defense solicitors.
There were concerns that evidence was not reported early enough.
One of the defendants affected was Liam Allan, 22 at the time, who had dropped when critical material emerged while he was on trial.
The Met Police is a great choice for Mr. Allan's series of errors in his case. He later told the BBC the matter had "completely ripped apart my normal personal life".
The CPS then launched a review of every live rape and serious sexual assault prosecution in England and Wales, along with a shelf, has been implemented to improve the fix fails in the system.
But Rachel Almeida from Victim Support said it was "very likely" that the police had accessed their personal information.
She said: "We know that rape and sexual assault is already highly under-reported and unfortunately this news could further deter victims from coming forward to the justice and support they deserve."
A legal challenge is already planned, the Center for Women's Justice said. A claim is expected to be brought by at least two women who have been told their cases could collapse if they do not cooperate with requests for personal data.
The Center for Women's Justice is expressed by "serious concerns" over what it calls "excessive disclosure requests" from the police.
"Most complainants fully understand why disclosure of communications with the defendant is fair and reasonable, but what is not clear about their past history (including any past sexual history) should be up for grabs.
"We seem to be going back to the bad old days when the victims of rape are treated as suspects", Harriet Wistrich, the organization's director, said.
The CWJ also warned of "deterrent effect on the reporting of rape allegations", giving the example of a woman to Olivia who recently reported rape to police.
Olivia said: "My phone documents many of the most personal moments in my life and the thought of strangers combing through it, it makes me feel like I'm being violated once again."
'Digital Strip Searches'
Big Brother Civil Liberties Watch said victims should not have "choose between their privacy and justice".
"The CPS is insisting on digital stripes that are unnecessary and violate their rights," the organization added.
Rachel Krys, co-director of End Violence Against Women Coalition, said:
"The reason for this is that the investigators too often focus on women's character, honesty and sexual history, despite the rules that are taken to prevent this, and the actions of the person accused.
"There is no reason for rape investigations to require such invasions of women's privacy as a matter of routine."
Scotland Yard said it recognised the "inconvenient" and "awkward" the nature of handing devices to the shelves.
"People who have been victimized and subjected to serious sexual assaults, for example, is an awful thing to happen to them and you don't want to make it worse by making their lives really difficult.
"But it is the offender, the way the law is constructed, we have these obligations, we have to find a way to get that information with a) of disruption and irritation and embarrassment to the person whose phone it is that we are dealing with. "