Victims of crimes, including those alleging rape, are to be asked to hand their phones over to police – or risk prosecutions not going ahead.
Consent forms asking for permission to access information including emails, messages and photographs have been rolled out in England and Wales.
It comes after a number of rape and serious sexual assault cases collapsed after crucial evidence emerged.
Victim Support said the move could stop victims from coming forward.
But police and prosecutors say the forms are an attempt to plug a gap in the law which says complainants and witnesses cannot be forced to disclose phones, laptops, tablets or smart watches.
Director of Public Prosecutions Max Hill said they would only be looked at where it forms a “reasonable line of enquiry”, with only relevant material going before a court if it meets stringent rules.
The digital consent forms can be used for complainants in any criminal investigations but are most likely to be used in rape and sexual assault cases, where complainants often know the suspect.
The forms state that victims will be given the chance to explain why they don’t want to give consent for police to access data, but they are also told: “If you refuse permission for the police to investigate, or for the prosecution to disclose 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 consent to having their smartphones and mobile devices examined is a big ask.
Most modern phones have more computing power than that which powered the first Nasa missions.
They contain photographs (sometimes intimate), emails and social media postings – (sometimes deeply personal, sometimes indiscreet) – not to mention text messages written in haste.
Many people guard the contents of their smartphones jealously and would regard a police examination as an invasion of privacy.
Civil liberties groups have raised concerns that victims may not report crimes if they fear their smartphone will need to be examined.
However, the police 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 defence solicitors.
There were concerns that evidence was not being disclosed early enough.
One of the defendants affected was student Liam Allan, 22 at the time, who had charges dropped when critical material emerged while he was on trial.
The Metropolitan Police apologised to Mr Allan for a series of errors in its handling of the case, in which he was wrongly accused of rape.
It is understood messages found on the alleged victim’s mobile phone included some saying what a kind person Mr Allan was and how much she loved him. There were also references to rape fantasies, Mr Allan’s lawyer Simone Meerabux confirmed at the time.
Mr Allan 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 and, along with police, has implemented an improvement plan to try to fix failings in the system.
But Rachel Almeida from Victim Support said it was “very likely” that letting police access personal information on their phone could add to victims’ distress.
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 access the justice and support they deserve.”
A legal challenge is already being planned, the Centre 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 Centre for Women’s Justice expressed “serious concerns” over what it called “excessive disclosure requests” from police.
“Most complainants fully understand why disclosure of communications with the defendant is fair and reasonable, but what is not clear is why 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 victims of rape are being treated as suspects”, Harriet Wistrich, the organisation’s director, said.
The CWJ also warned of a “deterrent effect on the reporting of rape allegations”, giving the example of a woman, referred to as 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, to try to use it against me, makes me feel like I’m being violated once again.”
‘Digital strip searches’
Civil liberties charity Big Brother Watch said victims should not have to “choose between their privacy and justice”.
“The CPS is insisting on digital strip searches of victims that are unnecessary and violate their rights,” the organisation added.
Rachel Krys, co-director of the End Violence Against Women Coalition, said: “We have an extremely serious problem with prosecuting rape in this country and it is a fact that most rapists get away with it.
“Part of the reason for this is that investigations too often focus on women’s character, honesty and sexual history, despite rules which are supposed to prevent this, instead of the actions and behaviour of the person accused.
“There is no reason for rape investigations to require such an invasion of women’s privacy as a matter of routine.”
The Metropolitan Police said it recognised the “inconvenient” and “awkward” nature of handing devices to police.
In a statement, the force said: “People who have been victimised and subjected to serious sexual assaults, for example, that’s an awful thing to happen to them and you don’t wish to make it worse by making their lives really difficult.
“But to pursue the offender, the way the law is constructed, we do have these obligations, so we have to find a way of getting that information with a) as much consent as we can, which is informative and b) with the minimum of disruption and irritation and embarrassment to the person whose phone it is that we’re dealing with.”