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 coming forward.
- Director of public prosecutions criticised over case failures
- Hundreds of cases dropped over disclosure failings
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 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 only relevant material going before a court if it meets stringent rules.
When could the police ask for your phone?
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 and there may be crucial evidence in their communication before or after the alleged incident.
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."
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 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 that might help them prove an accusation to be true, but also might help them decide an accusation is false.
She told BBC Breakfast: "None of us want to lose our phone for an hour let alone weeks or months.
"It depends on so many factors. Not just the handset, [but] the model, when you last updated it, how much material there is on there.
"We are also dealing, as we all know, with police forces with really quite limited resources here."
Analysis: Would you say yes?
By Clive Coleman, BBC legal correspondent
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?
Requesting 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.
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."
He added: "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."
Northumbria Police and Crime Commissioner Dame Vera Baird said it was a "real concern" people will be put off making a complaint if they expect to have to hand over their personal data.
How have other critics responded?
Harriet Wistrich, director of the Centre for Women's Justice, said: "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."
The charity 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 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."
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."
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