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 when crucial evidence emerged.
Victim Support said the move could stop victims coming forward.
But police and prosecutors say the forms can plug a gap in the law which says complainants and witnesses cannot be forced to disclose relevant content from phones, laptops, tablets or smart watches.
Director of Public Prosecutions Max Hill said such digital information would only be looked at where it forms a "reasonable" line of inquiry, with material going before a court only if it meets stringent rules.
Consent forms are not used in Scotland and Northern Ireland.
Police Scotland says officers only seek to access someone's phone if it is deemed "necessary and proportionate" and that all cases are assessed individually.
PSNI says its officers will only seize phones when it is deemed necessary and in accordance with legislation.
When could the police ask for your phone?
The digital consent forms can be used in any criminal investigations - but are most likely to be used in rape and sexual assault cases, where complainants often know the suspect, and there may be crucial evidence in their communication.
The forms state that victims will be given the chance to explain why they don't want to give consent for police to access their data.
But they are also told it they refuse permission "then it may not be possible for the investigation or prosecution to continue".
'My phone was taken for two years'
One woman, who wants to remain anonymous, says she was raped in April 2016 by someone she knew and reported it two months later.
"I was willing to give the police everything they needed. I'd texted people that night about the incident. The police said they wanted to extract data from my phone.
"I was required to hand in my phone and it was only returned to me after repeated requests after two years.
"When I got my phone back, I saw that it had not even been turned on in two years.
"I might lodge a complaint at some point, but I just felt everything was so invasive at the time.
"They didn't even take the phone off the perpetrator. I gave his name and address. He's not had to face any consequences."
'I didn't hand over my phone'
Another woman, Leah, who was a university student from London, was sexually assaulted on campus last summer.
"I didn't have the courage to report it straight away - but when I saw him again on campus, I had to.
"A policewoman called to organise an interview and told me she'd need my phone. I didn't see the need to hand it over.
"The whole thing was very stressful, and adding stress to what had already happened to me. I didn't think it was relevant.
"When I turned up at the interview and didn't hand over the phone, I was made to feel that I'd done something wrong. It felt so invasive.
"I got halfway through the interview and then stopped. It was almost as traumatic as the incident itself."
Why do detectives want access to mobiles?
When investigating criminal allegations, detectives might want to access digital communications when gathering evidence for a prosecution.
The move to introduce consent forms is part of the response to the disclosure scandal, which rocked confidence in the criminal justice system.
Several court cases of rape and serious sexual assault cases collapsed when crucial evidence emerged at the last minute, prompting 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.
Do victims really want to hand over their phones?
Critics of the move have said it could prevent victims, particularly teenagers, coming forward over fears they will have to hand over their mobile.
Obtaining an alleged victim's mobile can pose a "delicate balancing act" for police, says Kama Melly, a barrister based in Leeds.
They must weigh-up a person's right to privacy along with their need to diligently investigate all material.
What do the police say?
Metropolitan Police Assistant Commissioner Nicholas Ephgrave said he recognised it was "inconvenient" to hand over devices to police and admitted: "I wouldn't relish that myself."
But he said it was sometimes necessary to get information from a phone "with the minimum of disruption and irritation and embarrassment to the person whose phone it is that we're dealing with."
Northumbria Police and Crime Commissioner Dame Vera Baird said it was a "real concern" that people could be put off making a complaint if they expect to have to hand over their personal data.
Why critics are unhappy over 'digital strip-searches'
Harriet Wistrich, director of the Centre for Women's Justice, said most complainants understood why they needed to disclose any communication they had with the defendant, but not why their past sexual history would be relevant.
She added: "We seem to be going back to the bad old days when victims of rape are being treated as suspects."
The charity is already planning a legal challenge, supporting at least two women it says have been told their cases could collapse if they do not co-operate with requests for personal data.
Rachel Almeida from Victim Support said letting police access personal information on their phone could add to victims' distress and "further deter victims from coming forward".
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.