The National Police Chiefs' Council said Monday that police will only seek access to mobile phone data when it is necessary for investigative purposes, but the new policy has already sparked a backlash.
It means that victims of rape and other crimes who agree to provide access to their mobile phones, laptops, smart watches and other devices will have their emails, texts and photographs available to authorities.
The consent form warns that withholding access could lead to cases being dropped.
"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," the form states.
The use of the consent form may face a legal challenge from at least two rape victims, the Center for Women's Justice said Monday. The law firm said the victims were told by police their cases would probably collapse if they don't provide personal data.
The group said in a statement that the new policy is "clearly having a deterrent effect on the reporting of rape allegations."
Griff Ferris, policy adviser at Big Brother Watch, said the policy violates victims' rights and subjects them to a "digital strip search."
The police chiefs tweeted that "officers will only ask for access to private data when necessary and proportionate, and advice will be given so victims can make an informed decision."
Police said there would be stringent rules to avoid misuse of the information in court.
James Slack, spokesman for Prime Minister Theresa May, said police understand that the use of personal data in criminal investigations is "a source of anxiety." He said police will balance a respect for privacy with the need to pursue "all reasonable lines of inquiry."
The forms have been introduced after several recent sexual assault cases collapsed when crucial evidence emerged from mobile devices.
The independent charity Victim Support said in a statement that giving police access to all the personal information contained on a mobile phone is "very likely" to distress victims and may make them decide not to contact authorities.
"We know that rape and sexual assault is already highly underreported," the group said. "Most rapes and sexual assaults are committed by someone known to the victim, so if they have been in contact with their attacker, they may worry that their claim won't be taken seriously and be less likely to come forward."